It's called Muxtape and it is all the rage with the kids. As you can almost guess from the title, it is a site where music lovers share what used to be called mixtapes.
I can't figure out how it is legal or how it makes money but it is as simple as uploading 12 songs and presto!, you're done. The site is extremely minimalist with no search function for users or particular songs that I can find but it is elegant and easy to use.
I quickly put one together with some (mostly) recent rock songs that were already in my itunes but my mind is already racing ahead to lots of fun potential mixes I can make. Click here to check it out. You can click on Song One and let it play all the way through or skip from song to song if you prefer. Please enjoy!