This is soooo good. There’s a plug-in for Winamp called PaceMaker. It lets you independently control the pitch and tempo of an mp3 (and probably anything else winamp can play). I’m listening to "Born To Run" at half speed with everyone playing an octave higher. Broadcast plays at break-neck speed detuned to hell. Just Amazing. You have a whole new musical collection to amuse yourself with.
Install it, select it in Preferences -> Plug-ins -> DSP/Effects and a little window should appear. One slider controls tempo (50 to 200%), the next pitch (-1 to 1 octave shift) and the last is the more traditional speed control (50 to 200%) you might find on a turntable. This thing sounds best pushed to extremes, but it’ll cost you some CPU time and winamp may crash after awhile (trying to play a Vorbis file crashed it quick), but you can just open it back up.
2006-06-22: Updated Pacemaker link and it’s no longer nagware!