Wren - Offset your carbon footprint by saving rainforests
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Capture - A great free screen capture utility that allows you to capture either a window or the desktop and save it to either a file or the clipboard.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Treepoints - Fight climate change and earn rewards
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.