MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Annotate.tv - Optimize your learning on YouTube
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Simply.Digital - Goals & Tasks - Todo list to track your time & help set meaningful deadlines
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Slid - Capture knowledge from online videos, courses, and webinars