last.fm AMPtastes monitors what a user listens to in Winamp, and whenever a new track is played, last.fm is queried to suggest a similar track, which is then displayed.
features
Minimalist interface that only appears when a track is suggested
Does not require a last.fm account
Only supports Winamp 2+ at present
Quick copy button for pasting the artist and track name into the user's music store of choice