The Left 4 Dead 2 demo is now out there for public consumption, but users will have found that the game's signature versus mode has been locked. No problem. Just hack your way around it!
Modder Saul has whipped up a little plugin that allows you to play as the zombies in an online match (PC version only, of course). If you want to host a game, you download the plugin, if you just want to join a game, all you need to do is enter some commands in the console and you're good to go.
Interested? Full details at the link below.