This little cross-fader is a trip. Retail is $30, which is reasonable. It even has a headphone jack.
Even though it could never suffice as a proper DJ mixer -- especially since it doesn't include pitch control for beat matching -- it might do as a last resort.
If you're ever hanging out with friends drinking, and wish you had some DJ gear to get the party happenin', all you need are a couple of mp3 players and this sucker. Well, and some speakers.
Check out Gizmodo's little piece on it.
You can buy one at Amazon.