Mario invades Left 4 Dead in Mushroom Massacre.

I guess it was inevitable given the various amount of Mario styled maps fans already make for other games; but somehow seeing Valve's undead horde wandering through the Mushroom Kingdom (Super Mario 64) is rather unsettling!

Currently 25% complete, you can follow the progress of Mushroom Massacre on l4dmaps.com. The first part of the campaign is already there for you to download along with full installation instructions.