What happens when you bring together the joy of the past with the joy of the... well, more recent past? Mari0, a mashup of Portal and the original Super Mario Brothers.
Mario gets a portal gun and the game mechanics from Portal with four-player co-op in a scratch recreation of the Super Mario worlds. And if that's not enough, you can download extra mappacks and the level editor.
It's open source, free-as-in-beer, and available for Linux (as well as Windows and Mac).
Watch the trailer:
Watch the demo:
Then get the game from stabyourself.net.