File under "Things I Write for Ars Which Are Also Very Cool on Their Own:" MySong is a Microsoft Labs/University of Washington project that creates simple accompaniment for a vocal melody. You sing into a mic while it plays a metronome to keep you on tempo, then the software figures out a basic set of chord changes to go with your song. The chord choices can be tweaked using "happy" and "jazz" sliders, which is one of the best user interfaces I've ever heard about. If only Word had sliders for "happy" and "jazz" it would make my writing much more fun.
What's interesting about this is not that it's some kind of advanced technology--it's actually very simple, and could probably run on a cell phone. But if you watch the video on their project page, it looks like the kind of thing that's just naturally enjoyable--the audio equivalent of the Photo Booth that comes with OS X. People with media production experience tend to look down on "beginner" tools like Sequel or Garageband, but we forget that there's a whole class of people for whom even those tools are both difficult to use and unnecessarily ambitious. I know a lot of people who are not necessarily musical, but would still be delighted to sing a song to their computer and have it add a backing band, no matter how simple or generic.