Take user-submitted content and voting, a la Reddit and Digg, strip out the dorm room aesthetic, and apply it to music. You end up with Stereo8, a fun-to-use user-driven internet radio station.
Visit Stereo8 and select one of the three broad genres—they'll be expanding to 10 shortly—and begin listening. You can simply listen without participating, but if you want to shape the way others hear music you can login using Facebook Connect to vote music up.
If you want to participate even more than simply listening and voting, you can also upload music which will be inserted into the playlist for future play. The uploaded track will stay on the Stereo8 servers for up to 48 hours or until it is sent through the playlist once, which ever comes first — you can read over the fine print in regard to that arrangement on the Copyright page at Stereo8.
Stereo8 is a free service that requires no login to listen and a Facebook login to vote and upload.