A guy that I went to school with just launched a pretty awesome website called anywhere.fm. It's funny that I heard about this yesterday, because 2 days ago I was telling a friend that I could NOT think of a website that I would use that I didn't already have. anywhere.fm turned out to be pretty great and I plan on using it.
Basically, it's a flex version of itunes that runs in your browser. It's free, and you can upload as much music as you want to your account and then stream it anywhere by just logging into the site. They also have a bulk uploader you can install on OS X or Windows in case you want to upload a lot of music.
Finally, there are a few social features. You can have friends and listen to their music as well, but the catch is you can't see their playlists or choose directly which of their songs you want to listen to. You can just listen to their stuff in shuffle mode. (Honestly, I'm not even sure how that's legal.)
In short, this service looks pretty fantastic, but my only reservation is that there's no obvious business model for this. I have no idea how they will pay for the bandwidth and storage costs and keep the service free. However, it is free now, so check it out: