Blender is an open source 3D modelling software with built-in game engine, Bullet physics engine and fluid animation engine. Questions involving issues with Blender itself should be posted on the relevant Stack Exchange site, Blender.SE.
Blender is extremely popular among enthusiasts and has started to be taken seriously in production too. It has been used in the movie 2012 and numerous commercials. It is now maintained by Blender Foundation, which is a non-profit organization. Blender Foundation frequently undertakes development of challenging and high quality open source movies which allows accelerated development of Blender.
Blender Stack Exchange.
Stack Exchange's dedicated Blender site.