MusicStrands is a music recommendation site and system akin to last.fm. There's a plug-in that you add to iTunes that watches what you listen to, and forwards observations to MusicStrands' servers. Then new music is recommended to you. MusicStrands also incorporates tagging and playlist sharing, encouraging social stickiness.
Who knew that they had an applied research lab? They not only document whizzy new features in MusicStrands, but publis the occasional technical paper. Torrens, Hertzog, and Arcos have one on music visualization (PDF), while Baccigalupo and Plaza are applying case-based reasoning to playlist recommendation (PDF). Kewl!!