Just in case you weren't satisfied with the 2D MOBA goodness that is Awesomenauts, the developers have snuck a little bonus into its Steam release. There's a pixelated Smash Bros style fighter buried in the folders of Awesomenauts. It's pretty simple, with no real special moves to speak of, but can handle four players locally, so that's a fun little diversion. And who knows, we may see an update or two for it, if we're lucky.
As reported by Joystiq, you can find ROFLnauts by clicking through to the SteamApps folder, then Common, then Data. The exe(cute)able will be sitting right there, ready to play. And you know it'll be fun, because it was designed by Jan Willem Nijman and Paul Veer of Super Crate Box and Luftrauser fame. Ah, the joys of free stuff.