Old school gamers should really get a kick out of this one.
Someone has gone through the trouble of converting the original Left for Dead into an 8-bit NES game. So far they have converted the first campaign, 'No Mercy' with plans to convert all 5 maps of all 4 campaigns.
The game is scheduled to be made available as a free PC download Jan 4th, 2010. According to the game's creator, '([Left 4 Dead developer] Valve doesn't know about it yet, but I'm sure (with their love of community creations) that they will find this mildly amusing at worst, and hilarious at best.'