One of the first indie games we bought from XBL, and one of our house favorites, is I MAED A GAM3 W1TH Z0MBIES 1N IT!!!1.
It’s a twin stick shooter that is exactly 13 minutes and 37 seconds long, set to a song by James Silva about how he, well, made this game. And put zombies in it. It costs a dollar, and he hopes you will play. (From that link you can listen to the song, and download it for a price you name. After all, the game itself has the song and costs a dollar. The song alone is worth that.)
The game is short and hilarious and allows for four players, so we play it often. The game play is my favorite of all twin stick shooters I’ve tried. And since zombie games are so rare these days…it’s hard to pass this one up.