Art of Illusion is a 3D-modeller, renderer and raytracer. It is written in Java and runs on Linux, Solaris, OS X, Windows and probably more. http://www.artofillusion.org
MplayerXP is branch of the well-known mplayer which is based on a new, thread-based core. The new core provides better CPU utilization and improves performance of video decoding. http://mplayerxp.sourceforge.net/
"An Oscilloscope On Acid" - Cthugha also lets you save certain visualizations to keystrokes which makes it a great tool for VJs. http://www.torps.com/Cthugha/