After Looking at Pandora.com and lastfm.com I had a snoop around and found a few other sites worthy of a look.

Songza and seeqpod are good if your looking for that one tune and you want to listen to songs individually :

http://songza.com/
http://www.seeqpod.com/

SkreemR is supposedly "the worlds greatest search tool" I dont know about that.. but it seems interesting enough and quite possibly a worthy lil tool to have i guess.

http://skreemr.com/

A couple of other radio station based apps are finetune and Jango. Here you select your style and slowly pick and unpick the songs you like and it creates the perfect collection of music for you. Much like Lastfm and Pandora. One thing to note about pandora is that it will not work outside of the USA, sad to say cos that was the first application of this nature that I ever used and it was nice .

http://www.finetune.com
http://www.Jango.com
http://www.last.fm/home
http://www.pandora.com/

If your into watching videos then IMEEM might be of interest. you can watch videos and listen to songs too.

http://www.imeem.com/

NOW to top all of those site with a true WEB 2.0 experience is thesixtyone.
thesixtyone is an experimental game that supposedly transforms listening to new music into a fun, massively multiplayer adventure. Given that you can create an playlists, share music with others, subscibe to others playlists, give songs a bump all while getting points and levelling.
You can also add music and video.

http://www.thesixtyone.com/