MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Spotify - Map shows when two people play same song at same time
Amarok - The Amarok team is proud to announce the immediate availability of the result of 2 years of hard work, the beta release of Amarok 2. 9. This release is marked by improvements, corrections and polishing of nearly all Amarok features.
SoundCloud - Enjoy music & follow favourite artists
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Last.fm - The world's largest online music service. Listen online, find out more about your favourite artists, and get music recommendations, only at Last.fm