I found it because Amarok (nice audio player for Linux) has an option to connect to last.fm. It also has an option to report music you listen to from your personal collection, which helps the system to find other music that you'd like.
On top of that, it will show you your "neighbors" each week. Your neighbors are people with similar musical taste as you, which means you might want to check out their playlists or recent music.
All of this is free, and for a small fee you can have advanced features. Subscribers can create their own radio stations and have access to discovery mode which keeps track of music that you've listened to for a year and only plays you songs that you haven't heard before.
So far, these are the only downsides I've seen:
- The servers are under heavy load and sometimes time out. They've recently been bought by CBS which might mean a bigger server budget.
- They've been bought by CBS, which might mean your listening habits are being collected for evil uses.
- There are still several bands that aren't included in the library. Judging by other music on there, I think it's all user-uploaded so that will likely change. On the upload screen they specify that the artist should upload his own music, but I doubt that's the case.
- It can be hard to find how to add a song to your playlist. If you search for the song you can play a sample, but not playlist. The secret is to search for the band and playlist it from the band page or listen to it and then playlist it from your recent tracks.
