MikuMikuDance

Source Filmmaker (often abbreviated as MMD and often misspelled as Miku Miku Dance) is a 3D video-making software, which as the name implies, was originally designed so that people could make Hatsune Miku Dance to her music. Since then versions have been developed where may import any compatible model.

People can export motion data which can be re-used by other people and on different models, often to amusing results. This is how MMD is most commonly used, however it actually works as a video editing software similar to the likes of Source Filmmaker or Autodesk Maya, and both pictures and videos can be made, though it is only capable of rendering AVI video files.

The english release can be found here: http://www.geocities.jp/higuchuu4/index_e.htm

Use by Untergangers
There is no hitler model publicly available and most downfall parodies are made with stand-ins such as Hatsune Miku herself.

While Untergangers do not often use MMD, MetalLone WolfScoruge is the only known unterganger who does, supposedly opting to use it after receiving a copyright strike on one of his downfall parodies and Rise of Core videos, which also feature Downfall parody characters.

It is also possible, as with any other 3D model program or Garry's Mod, to chroma key characters into downfall parodies using the actual footage.