We had said, amongst ourselves, that we probably had enough Left 4 Dead coverage today already. This one's just far too epic not to report, though. Via Castle Crecente, it's Left 4 Dead played happily in Team Fortress 2 maps:
Man! Fast work, modders. There's safe rooms and everything. It also supports Hl2 maps, apparently - there's a guide on how to do it yourself here. So, within less than a day, there being only four official L4D campaigns has stopped being a problem. Happy Zombieday!