I just stumbled upon this site. A little like Pandora but with built-in streams as well. I'm really digging it so far
I've listened to the Adult Alternative station for a few minutes and haven't heard one bad song. So far it's been Radiohead, Modest Mouse, Ben Harper, Beck, k-os, and a few other artists I've never heard of but liked (The English Beat, Roxy Music, Sia) Seems like a little broader than I'd define "Adult Alternative", but that's fine by me.
If you don't like a song/band you can ban them or just skip them and never here it again on that station. You can pause it if you need. Etc. The streaming quality is pretty good.. I can't seem to find the bitrate but it sounds better than most stations. I'm guessing around 128kbps.
They also have a portable player, that you can either stream the songs directly over wifi or download to the players HD, coming out later this year.. which sounds really cool. It has a 4 inch (!!) screen and supposedly ranges in storage capacity from 2GB to 120GB, and price from $149 to $299. It'll also support regular mp3s/wmas.
http://www.slacker.com/
