Muxtape.com is a new web application which allows users to make online mix tapes by uploading MP3s, which then can be arranged into a "mix tape" people can listen to online. It gets bonus points for the interface, which has a minimal elegance about it and does everything other than the actual music playing in DHTML. On the down side, you only get to put 12 MP3s in your mix, and are not supposed to have more than one mix.
(For what it's worth, my one's here.)
I wonder which came first - http://www.mixwit.com