Originally pointed out me by Sexy Dexy, this is a great site, which has a surprisingly large array of possibilities.
[URL="http://www.finetune.com/"]FineTune.com[/URL] or [URL="http://www.finetune.com/wii"]FineTune.com on your Wii![/URL]
Essentially, it's a playlist site. You pick 51 tracks from a library of megathousands, and you can share it with anyone on the interweb. Might seem a bit pointless if you, like me, have zounds of music already on your hard disk and mp3 player. But, the opportunities present themselves - listening to the playlist at work, at school or - my own personal favourite - on your Wii, using the site's custon Wii browser. I think it'll double up nicely now as a music system for my living room.
It's also kinda neat for seeing what music other people like. My profile name is TheX.