Resistance movements were present in some form or other in every country occupied by the Axis forces in Europe, as well as within Italy and Germany themselves. Although some were small and disjointed, and others spent as much time fighting other resistance movements, they nonetheless made a significant military contribution during the war. Resistance movements disrupted, distracted and damaged occupying forces. Every German soldier sent to guard a viaduct in the Norwegian mountains, or gunned down on the streets of Paris, was one less fighting on the eastern front. Every hour a train was held up by sabotage on D-Day was an extra hour that Allied troops had to fight their way off the beaches.

It's difficult to make a qualitative judgement of the military contributions of resistance movements (Eisenhower is said to have judged the contributions of the French resistance as being worth an extra six divisions, although this number varies based on the retelling), but operating as a larger part of the Allied effort to defeat the Axis in Europe, they made a significant contribution.

It's worth noting at the start of this answer that resistance actions were incredibly dangerous - not just for the resistance fighters, many of whom were killed by the Germans - but also for civilians living close to where a sabotage attack or ambush had taken place due to the risk of reprisals. Following the assassination of Reinhard Heydrich by Czech SOE agents, the Germans murdered 340 civilians in the village of Lidice. In 1941, the Germans murdered 48 hostages in Brittany following the assassination of Lieutenant Colonel Hotz by communist resistance fighters. Such killings took place on an even larger scale on the eastern front - German soldiers in Belarus carried out 'anti-partisan operations' which killed far more civilians than partisans. For example, in one operation which took place in Autumn 1941 the 707th Infantry Division shot 3423 'partisans and helpers', losing only 7 men dead and 8 wounded. Max von Schenckendorff, commander of the rear area, remarked that "among those the division reported as being partisan helpers, there appear to be many who were only loosely linked to the partisans".

Active combat against the Germans by resistance movements and partisans often led to heavy casualties, as the movements lacked the heavy weaponry and training to be truly effective. When French resistance forces attempted to take and hold the Vercors Plateau in 1944, the Germans sent around ten thousand men to crush them, and they lost over 600 men against just 65 German losses. While resistance groups could make life both dangerous and difficult for the German and Italian occupiers, they were generally unable to take and hold territory against concerted attacks. While this limited their overall military contribution to the war, every soldier killed in a small scale ambush or shot in the streets of Paris, and every soldier subsequently brought in to tighten German control was a soldier who was taken from other duties.

Where resistance movements couldn't hold territory themselves, they were able to limit its usefulness to the German occupiers by means of widespread sabotage. In the west, resistance groups armed with explosives supplied by Britain were able to cause significant damage to industrial installations and railways. In June 1943, French resistance fighters destroyed the Fives-Lilles locomotive factory, and in July an SOE agent persuaded Rodolphe Peugeot to allow the sabotage of the Peugeot factory, which was making tracks and engines for tanks. In Norway, SOE saboteurs destroyed the heavy water plant at Vemork and sank a ferry containing the surviving heavy water stocks, causing a significant disruption to the German nuclear research programme. One of the first major acts of sabotage saw SOE operatives and Greek resistors demolish the Gorgopotamos viaduct, putting a major rail line out of action for 19 days. Resistance movements were also involved in hiding escaped prisoners of war and smuggling them back to safety. A number of sophisticated 'escape lines' ran from the low countries through to Switzerland or the Alps, with one line running into Brittany where escapees were picked up from a beach.

Resistance movements also made a significant contribution towards disrupting German movements and telecommunications both on D-Day and in the months afterwards. Famously, the French resistance caused significant damage to telecommunications and railway networks to delay reinforcements reaching Normandy. Danish saboteurs delayed one division by six days through a sustained sabotage campaign. As the Allies neared the German border in the West, the Norwegian resistance undertook a sustained campaign of sabotage to slow down German troops leaving the country. Almost fourty major attacks were carried out, including one operation involving over a thousand men and resulting in the destruction of ten railway bridges, and a separate attack which destroyed the headquarters of the Norwegian state railways in Oslo. In the east, vast numbers of partisans were involved in attacking German communications, isolated garrisons and supply lines in the run up to major Soviet offensives and large engagements.

As the Germans retreated, resistance movements were able to carry out 'counter-scorching' operations, preventing the Germans from sabotaging infrastructure and industry as they left. In Norway, resistance fighters stole a number of ships the Germans planned to sink in major harbours to block them and sailed them to Sweden. In Bordeaux, resistance fighters kidnapped the wife of the German officer responsible for destroying the radio broadcast station and negotiated her return in exchange for leaving the station intact. In the ports of Nantes and St Malo the resistance was able to sabotage some of the demolition charges, reducing the eventual damage to the ports.

Within Germany itself, there was not an organised wide scale resistance such as the one seen in occupied France. There were intelligence networks such as the Rote Kapelle which fed military information back to the Soviets, unaffiliated groups such as the White Rose Movement and Edelweiss Pirates and a significant opposition group within the German officer corps, but there was never widespread popular resistance to the regime. Perhaps the most striking military or strategic contribution to the war's end were the various attempts to kill Hitler. Acting alone, Georg Elser came very close, failing to kill his target only because Hitler left unexpectedly early. The resistance to Hitler within the German armed forces came close on a number of occasions. On a visit to the eastern front, a group of plotters in Army Group Centre smuggled a bomb onto his plane, which failed to explode for unknown reasons. On a second attempt, an army officer wearing a suicide vest was due to blow Hitler up during an inspection of captured Soviet equipment. Hitler however rushed away before the bomb could be detonated. Most famously, the plotters managed to detonate a bomb in Hitler's presence on the 20th of July 1944 but a combination of a smaller than planned explosive charge, the movement of the device within the room and a changed location meant that the explosion failed to kill Hitler. The plotters launched and bungled the subsequent planned coup and were executed by the Nazis.

Whether the removal of Hitler would have made any contribution to the war militarily is somewhat up in the air. It's a commonly held belief that the British didn't want to kill Hitler because his tactical blunders would significantly shorten the war. However, while a meeting of the Joint Chiefs of Staff in 1944 decided that "from the strictly military point of view, it was almost an advantage that Hitler should remain in control of German strategy, having regard to the blunders that he has made [...] on the wider point of view, the sooner he was got out of the way, the better", the British nonetheless were developing plans to kill him.

So in conclusion - resistance movements made significant contributions to the eventual military victory during the war, whether by sabotage or carrying out attacks on armed forces. While not a decisive role, they nonetheless made a significant contribution to easing the pressure on both the western and eastern front. However, sabotage and armed attacks often caused the Germans to carry out brutal reprisals against the civilian population.