I tend to use Grooveshark when I use online music services. I'm more into indie/folk rock, but throw in whatever you like. I think what I'm going to do is require it to be a playlist that is around or less than an hour long. I'll create a thread if we can get more people in on this.
EDIT: Another good idea would be to have everyone send their mixes to me, then I could post them in a thread so that everyone can listen to each other's music.