Free games on DSiWare are usually not what I would typically consider "games." There are some clocks on there, and some sketching programs, and a chat tool, but nothing very game-y. "Photo Dojo," however, is definitely a game. It's gimmicky, sure, but it's sure to entertain for at least 30 minutes, and what more could you ask from free software?
Conceptually, "Photo Dojo" is a lot like "Mortal Kombat" without the art. Basically you are asked to provide the different fighters in the game using pictures taken by the DSi. The game provides frames for your friends to stand in (like a punching frame or a taunting frame) and strings all your pictures together to make a (hopefully) animated 2D fighter.
It's one of those games that is only as creative and interesting as you want it to be. Theoretically you could make your dog one of the fighters, so long as he doesn't mind being held up in a kicking pose. You could then battle your neighbor's dog in the ultimate PETA-friendly dogfight.
"Photo Dojo" is up on the DSiWare store now and will be free for the next month, at which time it'll cost 200 Nintendo DSi points. So yeah, make like a tree and go download it.