Terry Cavanagh (VVVVVV) has released ChatChat, a free multiplayer Flash game and chat client in which the player's only instruction is to "be a cat."

It's ridiculously cute and charming. Controls and actions are explained within the game world, and the scoring mechanics should become obvious after a few minutes of play. Sure, it's not the deepest game in the world, but it's certainly the most realistic cat simulator I've ever played.