I never realised how easy it is to remove the vocals from songs (useful for karaoke or for making bootlegs). Just take the free, cross-platform audio editor Audacity and load in your MP3. Split the left and right channels, then invert one of the channels. Make each channel mono, then use the Quick Mix menu to put the channels back together. The theory is that anything that's the same in both channels (ie in the centre of the stereo mix) will be subtracted, and this is normally where you find the vocals.
From the Audacity FAQ.
See also AnalogX vocal remover, a Winamp and DXi plugin.