Number 1: moretrackstrackslikethis.com
A useful mashup of lastfm’s “similar tracks” service and Spotify gives a great way to find new artists and music matching your tastes. What’s particularly good is that you can match single tracks, so even if you only like one track by a certain artist, you can find good matches.
Number 2: admission that this is just shameless link-bait
There’s some other great mashups out there. Check Spotify’s list.