Muxtape is a wonderful new service created by Justin Ouellette that makes it really easy to create mixtapes, including a unique link that you can give out to friends.I made one for some of my favorite songs.
kittie.muxtape.com
Also, I made a muxtape of Barney's "Get Psyched Mix" from the classic How I Met Your Mother New Years episode.
Here is a link to a wiki list of other good muxtapes
