Spine - Spine is 2D animation software designed specifically for software and game development.
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
CrazyTalk Animator - CrazyTalk Animator, the next level of 2D animation. Create actors using any photo or illustration.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Synfig Studio - Just another WordPress site
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.