It's a fighting game in the style of Streets of Rage and Street Fighter, except you create your own characters using the DSi camera and mic. The game guides you through posing for various fighting moves, and has the ability to take pictures using either camera. After taking about a dozen photos, you are then stepped through recording the sound effects for punches, kicks, fireballs, etc. You can create up to 8 characters.
The single player mode is basically just a survival game, kill as many enemies as you can. Move right, fight enemies, move right. The enemies are chosen from among the characters you've created, so the more characters, the more fun and bizarre the game gets (especially if you let your friends make their own characters).
Vs. mode has one of the strangest control setups I've even seen. Instead of using two DSis, you just use the one, one player moving with the D-pad and attacking with the L button, the other player moving with the ABXY buttons and attacking with the R button. Game play is similar to Street Fighter, though it's a one-round winner-takes-all battle.
The game play is extremely simple, but the fun comes from creating the characters then watching them fight.
And of course, it's free. Free is always worth a download.