Muxtape seems like a great idea and just what I've been looking for: a very simple web-based mix-tape making machine. I'm tempted to complain that is doesn't have certain features (like, say, a search feature), but that would be an argument against its simplicity, which I love. I'm a little doubtful they'll survive the lawyers of the music biz, but here's hoping. My first muxtape is here. There will be more.
(For more simple, useful web apps, also see Instapaper and Ta Da Lists.)