Playlist.com

For a while I tried to get into Last.FM, but I never really stuck with it. I use Hype Machine for some songs, but the main way I stream music that I don't own is off of Playlist.com. The site might be covered in ads, but it works really well and the playlist creation and management tools are pretty good. Hype Machine has the better interface, but it's better used for finding new music or listening to a single artist's new stuff.

I've used Playlist.com for a couple years now, off and on, and I just started putting together playlists again. You can find my profile here, and listen to a few of them. The "New Wave" and "Party Time" lists are my favorites - also the newest, so that makes sense.