Ok, so two of my favorite online music sites so far have been Groove Shark because you can make your own play list. Be warned that it has a bit of a P2P situation going on, but it only works off what has been uploaded.

Also, check out Pandora which suggests music for you based off of the type of music that you like. Much easier if you just want to listen to something and not make a huge ongoing play list because that can take up time. Also be wised you can only skip a certain amount of songs per hour.

I love Playlist.com for myself since it will let you make multiple playlist and you can have the option of letting your friends add music they want you to listen to to your playlist. The one drawback to it is that it does not work internationally, some countries outside of the US have access but not all. So if you have a lot of international friends that you want to share music with it is not the best. Groove Shark is definitely better for sharing your fave tunes internationally.