Wren - Offset your carbon footprint by saving rainforests
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
YAYZY - Track the carbon footprint of each purchase in real-time
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Klima - Go carbon neutral
Kauna - Kauna is a music visualizer. Kauna renders any sound reproduction in system.