Sessian allows you to use your browser like a mixer, with an important caveat: the tracks can only be soloed or muted, not volume-adjusted, as far as I can tell. Still, turning your web browser into a primitive mixer is a neat accomplishment and a sign of things to come. The developer, Chris Whamond, goes into a little detail on his site.
Before coming to Rails, Chris also did a bunch of interesting direct marketing work, and was kind enough to do an interview with me last year, which is unfortunately sort of trapped on a semi-fragile hard drive. I'm going to upgrade my old box to an SSD drive, maybe two, and I'm hoping to blog Chris's interview once I get things cleaned up a bit.