We already knew that killing zombies was popular, but Ska Studios (best known for The Dishwasher: Dead Samurai) and their I MAED A GAM3 W1TH Z0MB1ES 1NIT!!!1 (yes, that's the title) have also proven that it is possible to sell a decent amount of a game on Xbox Live Indie Games.
In fact they have announced that it have sold in excess of 200,000 copies of the game (for a buck a piece). Pretty nice for a one man operation.