I recently got an account at last.fm and I can't believe I didn't do this sooner. I see people with their most recently played list all the time. In about a week, I've found at least two bands that I can see myself really get into: The Evpatoria Report and Esmerine. Check out my profilehere. My only complaint is that the site's update schedule is very irregular and slow. I know they can't afford to re-compute the rankings in real time, but that would be awesome.
Using last.fm and seeing other social network inspired projects (delicious, flock, etc) has made me think about other applications with social networks inside. The TiVo recommendation system and the Amazon recommendation list can definitely learn something from these sites. The feature of obscure vs. popular recommendations on last.fm is really neat. Also, instead of hiding where/who the recommendations come from, letting me check out other users' lists made the experience much more grounded. It also caused me to waste hours of just browsing the site.
I also found out about 4flix.net today via Digg (yet another great social website). You can buy classic movies on 4flix for $2 and there's no DRM! I saw a bunch of Hitchcook movies on there; will definitely need to get them later.