Well here are six sites that I've either recently discovered, have recently improved or continue to deliver as promised.
Deezer - French based site with a wealth of fully streamable CDs. Things are very hit and miss with many releases not available and some with only 1 to 4 songs available but a little patience pays off. High social factor as well.
Songza - This site strips audio off youtube videos and allows you to save playlists in a unique but effective user interface.
Slacker - Despite the lackadaisical moniker, this site seems to be a real up and comer. Think customizable internet radio with a slick interface and mobility options. High profile mention in Paste magazine and tons of praise for its new hardware.
last.fm - I have complained about this site in the past and at times could not understand all the praise heaped upon it. But with recent additions of free full track streaming and a serious UI overhaul, last.fm is really starting to live up to the hype.