As for Finland, the short answer is that most Finns were not that happy to join forces with the Nazis, but the geopolitical situation practically excluded any other option, and of course there were considerable numbers of Nazi sympathisers and even outright nazis. In the past, Finnish historians and politicians have generally argued that Finland and Germany were not allies but merely "brothers in arms" against a common enemy, but these days it's generally accepted that the arrangement looked like an alliance, walked like an alliance and quacked like an alliance. Finland's military leadership essentially committed to operation Barbarossa by the early 1941 at the latest, and there was considerable joint planning between the headquarters.

However, it is good to understand the reasons behind this alignment. After the Winter War, in 1940, the Finnish political preference was an alliance - in fact, a de facto union - with Sweden. However, the Swedes (probably wisely) made this conditional on the acceptance of both Germany and the Soviet Union. Both rejected the idea: Hitler saw Finland as an useful ally in the coming showdown with Stalin, and Stalin wanted to correct the failure of the Winter War. In early summer of 1940, Soviet divisions began to mass near the new Finnish border, which had lost the key defensive lines that had helped to stall the attack in 1939; a Finnish airliner was shot down by Soviet fighters with the loss of everyone on board; and other indications suggested an attack could be imminent. Soviet Union's foreign minister Molotov travelled to Berlin, asking for a free hand to deal with the "Finnish question"; however, at this point, Barbarossa was already being planned, and Hitler, who had withhold Italian arms shipments to Finland during the Winter War and given Stalin a free hand in 1939, send Molotov packing.

At this point, after the German occupation of Denmark and Norway, Finland was practically cut off from the Western allies. The only practical source of both weapons for the depleted army and food, a crucial item because Finnish soil could not support the entire population, was Germany. And these, of course, came with strings attached. German army would take over Lapland for a staging area for an attack towards Murmansk, and the Luftwaffe would use Finnish airfields to recover and rearm planes that attacked the Soviets in the initial stages of Barbarossa. The Finnish army was committed to an attack in Karelia, pinning down Soviet forces and recapturing lost territories (and more); the Svir (Syväri) river running to the eastern shore of Lake Ladoga was the objective where they were to link up with the Germans advancing from south.

Finland however waited until Soviet planes had bombed Finnish airfields in reprisal for their use by the Luftwaffe, thus providing a casus belli, and only then declared war on the 25th June 1941, three days after the start of Operation Barbarossa. The attack was largely supported by the populace: they felt that with the help of Germans, who had seemingly effortlessly crushed not only the Poles but the strongest army in Europe (the French), captured Denmark and Norway, and sent the British reeling, Finland would at the very least regain the prosperous territories lost a year before, and deliver the "ancient enemy" in Moscow the final blow.

However, as the Germans became bogged down and it became clear the war wouldn't be a short affair, opinions soured. Already in 1941, there were instances of small scale and even mass disobedience in the army, especially among units composed of reservists, who flat out refused to cross the "old border": they were all for kicking the Soviets from their lands, but did not want to risk their lives for the megalomaniac dreams of Hitler or of Finnish hardliners who wanted to build a "Greater Finland" from the territories of Soviet Karelia. By early 1942, the Finnish advance was over, and the troops began to dug in. Finnish high command, already skeptical of Germany's prospects, refused to assist in the siege of Leningrad (that field marshal Mannerheim had been a an Imperial Guard officer in the Czar's court there may have been a small factor), and rejected the idea of cutting the Murmansk railroad, which was a vital conduit of Lend-Lease aid shipped from the U.S. and Britain. The Allies had made it very clear that moves to this direction would result to very unpleasant repercussions after the war, and in any case, the attack was calculated to be very costly in terms of casualties. Since the short "attack phase" in 1941 had already cost about the same amount of casualties as the entire Winter War, this was not an appealing prospect.

By and large, there was rather little support for the Nazis during the war. There were Finnish Nazis, fascists, and conservatives who sided with them, but they had peaked in 1933 after their failed attempt at a coup d'etat, and did not participate in the government. Then there were some hardliners who were not Nazis but opposed peace with the Soviets even after it was clear in late 1944 that the alternative would be eventual occupation. In the end, Finland negotiated a separate armistice in the autumn of 1944, after the Soviet offensive that was supposed to break the Finnish lines and occupy the country had failed. When the war in Europe was over in May 1945, huge crowds gathered to celebrate, and communist and socialist parties gained a massive victory in the first post-war elections.

All in all, I believe it's fair to say that Finland was a de facto ally of Germany, but driven to this alliance mostly by the force of necessity. These circumstances were recognized even during the war by the Western Allies, and U.S. foreign department archives have revealed that the U.S. government position was that Finland, alone among all the allies of Germany, was to be permitted the freedom to choose her own government after the war.