I just read about this nifty idea on Xboxic.

There's an Xbox Live Arcade came coming out soon called Small Arms. One of the game's achievements is called "Six Degrees of Small Arms", and it's a unique achievement in the Xbox Live world.

The idea is that four guys who work at Gastronaut (the game development studio) will start off with this achievement. Anyone who plays Small Arms against them online automatically gets the achievement themselves. Then, anyone who plays those guys gets the achievement too.

It's a viral achievement. Very interesting - it'll be cool to watch how it spreads (if it spreads at all) as a gauge of how people play each other online.