The lights are on
Whether it's Half-Life's well-polished single-player experience, Left 4 Dead's intense multiplayer mayhem, or the amusing puzzle-like experience that was Portal, it's hard to argue with Valve pedigree, but you can certainly try if you'd like.
Allucard on Newgrounds.com made this awesome trailer of a mario left-4-dead crossover. I'm hoping he makes a actual co-op game out of it, it looks really cool.
This is a customized map for the original Left 4 Dead on PC. It is mapped after the Princess Peach's castle from Mario 64. It is truly awesomeand very nostalgic.
Wow that's just amazing. Just look at the detail the mapper put into it, because usually map copies have different props and textures built into it. I would love to get my hands on that map.