I can't speak for the millions of them, but a number were sold as surplus and made it to Mandatory Palestine, where they were used by both Jewish/Israeli and Arab forces in the war there. This photo posted here yesterday shows Arab forces (exactly who we discussed here) using a variety of WW2 weapons, including an American Browning M1919 machine gun, a German MP40 and P38, and a French rifle. Israel, too, often used a hodgepodge of surplus from Europe, partly because weapons were smuggled in during the British mandate.
I know this is just a partial answer, which doesn't address the issue of small arms and other military hardware, but a lot of bombs ended up in basements and buried under roads and apartment blocks, particularly in the South of England and in German cities.
About 10% of the millions of bombs dropped in the Second World War were duds. To put this in perspective, over 40,000 tons of explosives dropped on Berlin never detonated, and there are still thousands of pieces of ordnance buried under German cities. The Germans still discover more than 600 tonnes of munitions every year. In the UK, 15,000 bombs were found during construction between 2006 and 2009 alone.
These weapons are actually becoming more dangerous now due to corrosion and rust, which means controlled detonations will probably become more common. Many of them simply can't be defused at this point, as the stabilizing agents in the bomb decay over time, while the TNT remains viable.
Anecdotally speaking, I worked on a couple of urban archaeological digs in the UK, where an on-site bomb disposal expert is a common sight. We were told to expect German incendiaries and primtive cluster bombs while digging. There were even time-delay bombs that are particularly dangerous today. It's quite chilling stuff.
Many stayed in use for the years to come; just because the war ended did not mean the weapons became all of a sudden useless. In Europe it was important to the US and Britain that communist movements beyond the Iron Curtain were stamped out while the Soviets wanted to see them spread. Greeks, Italians, French, Belgians, Dutch, and other nationalities made good use of Allied WWII arsenals in the post-war environment. Some German weapons even made their way to Israel while the Arab nations fought with a variety of kit from Britain, France, etc. There are dozens of other conflicts (read: every conflict for quite some time afterwards) where the majority of weapons used were products of the Second World War: the Congo Crisis, the wars in Algieria and Vietnam, the insurgency in Malaya, the Indo-Pakistani conflicts, the Chinese Civil War, the Korean War, all the way into the present day. I viewed a video on /r/combatfootage just the other day of Croatian troops in 1991 in which I spotted a German MG-42 (as in 1942).
Many weapons also just went into storage. I know the US kept lots of small arms around 'just in case', as did the Soviet Union. American small arms also found their way into civilian hands through initiatives like the Civilian Marksmanship Program or you know, a Sears catalog. Virtually every Latin American military was equipped with American WW2 kit for the remainder of the 20th Century. The Cuban Revolution was fought with Jeeps and M1 Carbines and Bazookas. In the same way the Soviets had more than enough to equip the Eastern bloc forces in the early post-war years with T-34 tanks and MiG/Yak fighters. And not just the European satellites either - North Korea and China certainly were not producing everything they used in the Korean War.
Even Japanese weapons, obsolete during the war, were used extensively in Asian conflicts for decades.
I'm not sure off the top of my head, but I would guess that many artillery tubes, aircraft, tanks, and other heavy weaponry of the Axis powers were simply destroyed in battle or disposed of after the war. I'm sure tons of weaponry belonging to the victors was also scrapped. For instance, all the stuff the Allies brought with them to North Africa likely never left (for a modern parallel, look at how the US basically 'scuttled' billions of dollars worth of equipment in Iraq and is going to do so again in Afghanistan).
Old ships from the uncompleted German aircraft carrier Graf Spee* to the remnants of the Japanese fleet to those USN ships that barely survived made for great target practice or nuclear bomb test subjects. The last Fletcher class destroyer, a US design dating back to 1939, was finally decommissioned by the Mexican navy in 2001.
As a final note, many weapons of the Second World War are damn near mature technologies. Sure, the bolt-action rifle was getting to be outdated, but the M1 Garand was a hell of a weapon. The MG-42 is the basis for many modern machine gun designs. There's nothing wrong with a Walther P-38. A Spitfire, a P-51, a late model Bf-109 with a good pilot - these are the best non-jet fighter aircraft there ever were! It may be a slight exaggeration, but it's my understanding that the Super Tucano, a Brazilian-made turboprop plane that may become the standard in counter-insurgency air operations, is basically a souped-up Mustang (I'm clueless on the difference between a turboprop and a piston though). You get what I mean by mature technologies, though - a lot of shit made during the war didn't need to be replaced by a new design for a long time. Of course, tons of stuff did during the Cold War. The tank, the jet, etc. - these things were not mature. But the average 100+mm artillery piece, the average mortar or land mine, the machine gun, and many other pieces of military hardware haven't changed all that much in seventy years.
*edit: Graf Zeppelin, as 69f1 so kindly pointed out
In many cases, volatile chemical weapons were disposed of through the "CHASE" procedure ("Cut Holes And Sink 'Em"), where thousands of tons of munitions were simply dumped in the ocean off of coastlines around the world. This practice was commonplace throughout the twentieth century and continued well into the Cold War. In addition to chemical weapons, massive amounts of explosive ordnance (everything from hand grenades to large bombs) was disposed of in this manner. The weapons came from both surplus Allied stockpiles and also from captured enemy weapons.
There have been a plethora of incidents related to this practice in recent years, in which fishermen and marine life have been injured or killed after encountering live ammunition or through being poisoned by chemicals leaking from discarded weapons.
Article about US dumping of weapons off Okinawa (mentions 6,600 tons of captured Japanese chemical weapons being dumped): http://www.japantimes.co.jp/news/2013/07/27/national/a-drop-in-the-ocean-the-sea-dumping-of-chemical-weapons-in-okinawa/#.Us9Nq_RDuSo
Report on consequences of postwar weapons dumping: http://www.kimointernational.org/DumpingatSea.aspx
Report with interactive map: http://cns.miis.edu/stories/090806_cw_dumping.htm
Americans gave them to whoever would not be communist as part of the Marshall Plan. European nations used surplus US equipment for years and years. Even in the '80s, old M48 Patton tanks were still being used by NATO nations. Many of the hundreds of thousands of aircraft were scrapped unfortunately. The Russians placed the millions of their rifles in underground storage. They've only recently come tto light, being imported to the USA and sold for $100 a piece. German Weapons were strewn about the allied nations. Many of the K98 rifles that appear on the American market are actually ex-russian capture. They had a new finish put on the stock, and the swastikas marked out. Japanese rifles were destroyed en masse. Many of the american rifles that weren't given away overseas were also sold back to the american public for CHEAP. The old M1 Garand rifle used to be sold in catalogs for extremely low prices. M1911A1 pistols would go for $50 in a sears catalog. People bought these guns, and treated them like they were cheap. They were beaten up, and left to sit. Because the supply has dried up, but demand has increased exponentially, the M1 commonly goes for $900 and a period 1911 goes for around $12-1300.
It's kinda sad, but that's what happened to most of the small arms.
WWII surplus end up in a surprising number of places. One of the most interesting to me is their use in avalanche prevention.
Major ski areas still use WWII surplus howitzers to trigger controlled avalanches. It's not uncommon to see signs warning of unexploded ordinance when hiking around these places in the summer. This connection isn't completely surprising since the US ski industry has a very close connection to the 10th Mountain division. A division of ski troops (existing to this day) trained for winter mountain warfare during WWII in response to the fighting that took place in the Alps during WWI. These soldiers saw artillery used to trigger artificial avalanches (sometimes offensively), than returned home and founded ski resorts, like vail.
Avalanche Control (discussing a resort which uses Korean war artillery): http://www.nytimes.com/2009/03/20/us/20alta.html?_r=0
One interesting case was a C-47 crash in the arctic on 9 October, 1949, which you might have seen on the Top Gear Arctic Special a few years back. The story goes that ice was building up on the wings, and the plane was overloaded, but the pilot didn't think it was necessary to de-ice the wings. It got off the ground twice, but couldn't keep altitude, and finally came down hard on its gear and skidded to a stop.
As it happened, the US had so many C-47s sitting around from WWII that they just shrugged their shoulders and left it there.
Here's some more info, including a news article and the crash report: http://www.oldwings.nl/content/c47_yic/c47.htm
As newer, more advanced weapons became available, numerous small arms were sent to allied nations. The U.S. kept most of its M1 Garands in storage and used them though the Korean War and the beginning of Vietnam. Many of these were sent to South Korea where they remain in storage along with large quantity's of M1 carbines (South Korea wants to get rid of them and there are many in the U.S. who want to buy them but the State Department has blocked this)
The Soviet Union shipped its main rifle (see /r/MosinNagant) all over the world. The Mosin was adopted by most of the eastern-bloc at one point and rest of Asia used it at some point or another. Wikipedia's list of users! Many Mosin's became insurgency weapons simply because they are simple to operate, long ranged, easy to maintain and they have readily available ammunition.
Anecdote warning here, but I grew up in Nicaragua, and during the Somoza dictatorship, the Guardia Nacional (the de facto army and police) used American WWII surpluses, like the M1 Garand. There's even a Sandinista song about the Garand. They were actively used even after the fall of the Somoza regime, as late as the 1980s.
So here you have a example of WWII weapons supplied to a Latin American military regime.
During the war the USA supplied Britain's Fleet Air Arm with a lot of combat aircraft - Avengers, Hellcats, Corsairs, Wildcats - under Lend-Lease.
Part of the deal was that once hostilities were over these had to be handed back or destroyed.
In a few cases, the British carrier crews just pushed the aircraft over the side and into the sea. Presumably there's patches of the Indian ocean littered with Hellcats.
Also, U-Boats captured by the Allies were taken out to deep water and scuttled: http://ahoy.tk-jk.net/macslog/OperationDeadlight.html
Is it possible to measure the influence the surplus weapons had on the post-war conflicts and de-colonization wars?
I know that exactly one of those weapons ended up in my gun safe.
I know a lot of gun folks who have one or greater in their safes as well.
My love for my Springfield can't be underestimated.
My wife's grandfather and his battle buddy, both WW2 Cav scouts related that on the way back to the US after the Euro victory and Japanese surrender, a lot of equipment was dumped overboard into the ocean because there just wasn't a need for it.
Many airplanes were transferred to other countries, or sold for a pittance. Some US airplanes overseas were sunk (especially off carriers), buried or otherwise destroyed.
Tens of Thousands were flown to US boneyards and scrapped. Studies showed that it was cheaper to salvage and melt than to dismantle for parts.
Relevant ref.
Similarly, many ships were converted, or sold off,; some were used for target practice. A large number were mothballed; some few to be re-activated. The US had established national defense reserve fleets or ghost fleets of mothballed ships at multiple locations; the [james river fleet ] (http://www.abandonedcountry.com/2013/06/03/the-james-river-reserve-fleet-the-last-of-an-armada-that-served-the-world/) alone had a peak of over 850 ships. A few were reefed, or re-activated/converted, most were scrapped.
Both conventional and chemical weapons were sunk at sea in operation CHASE (Cut Holes And Sink 'Em.) and similar exercises.
And of course some weapons everywhere were just left to rust and rot.
Most airplanes were sold for scrap over the next years, a lot of them brand new or never used in combat.
Here's an article with more details, numbers and impressive pictures about the airplane side of things: http://www.planesofthepast.com/aircraft-boneyards-storage-after-wwii.htm
I remember a veteran B-17 crew member on a ng saying they were offered to buy their plane for a few hundred dollars, but the cost of the fuel, repairs and certification to fly it back from the UK was just too much.
At least some of the UK and German stuff probably ended up in a trench in the English Channell called the Hurd's Deep. The UK apparently used it to dump military stuff until the 70s.
According to this wiki article they also used the Beaufort Dyke.
I can only speak about airplanes really. Basically, most planes that were considered surplus after the war were sold off. Many bombers became fire planes or search and rescue. Hard as it is to believe, few people even wanted to buy them, so that vast majority of B-24s for instance were just scrapped. Fighter planes were also sold, often to private owners to become crop dusters or race planes. Some of these planes were still brand new, not even unpacked from their crates. In fact, a fair number of P-51 Mustangs and other fighters are still in use in races like the Reno Air Race. Let's just say, if I could go back in time to buy one thing, a freaking brand new crated Mustang might very well be on the top of the list!
A lot of the surplus weapons used in the pacific theater were simply thrown into the ocean. Edit: I am referring to small arms.
I've heard about English surplus rifles being used as fence posts, does anyone know if this is true or not?
So from the top comments what I can infer from this thread is that ww2 weapons went to Israel?
