iPod Mixer from Newmark
Time Magazine reviews...
"Take away scratching, looping, and tempo synching and what have you got? Say you're going to DJ a party. With two iPods already docked and charging, you don't have to worry about plugging in extra power or audio cables. You can use the trackwheels on the iDJ to easily navigate the contents of your iPods. You can preview one song while the other is playing, and independently adjust the treble, midrange and bass of each.
Provided you have an iPod Photo or any newer color-screen model, you can run a slideshow out the S-Video output and onto a TV or projector you bring along. It only works with the iPod in the left-hand dock, however—it can't blend photos from two iPods, the way it blends their sounds."
(Thanks to Ron from The Green Ant)