Blender is a free open source software for 3D modeling, animation, rendering, post-production, interactive creation and playback. It lets users create high quality 3D graphics, movies and replay real-time, interactive 3D content. Over the last decade, Blender has evolved as an in-house tool for a leading European animation studio into one of the most powerful open source 3D content creator.

Available for all major operating systems under the GNU General Public License, it has already been used in various movie productions and games alike. For those aspiring animation amateurs, this software gives you a chance to let your imagination run wild and show it off to the world with the same animation tools the professionals use.

Support platforms are:

Windows 98, ME, 2000, XP
Mac OS X (PPC and Intel)
Linux (i386)
Linux (PPC)
FreeBSD 5.4 (i386)
SGI Irix 6.5
Sun Solaris 2.8 (sparc)

Source [Blender]