Are you a Left 4 Dead fan? Do you love burrowing into a game's directories on your PC and modifying files? Then this somewhat complicated tutorial is for you. A group of modders have figured out how to import the first game's campaigns into Left 4 Dead 2, and it will only take owning both games, some technical know-how, and quite a bit of patience.
Since Valve has yet to release the SDK for Left 4 Dead 2, the process of modding the game is a little more in-depth than usual. You can find a full tutorial of how to make the process work on L4DMods.com (via Kotaku), but be aware it'll involve a lot of manually editing file names and transporting certain files from one directory to another. And even then, to get it all working you're going to need to fiddle with the command console in Left 4 Dead 2 (although honestly, that'll probably be the easiest part of this process).
So what do you get for all of your troubles? The option to play through No Mercy, Death Toll, Dead Air, and Blood Harvest from the first game, with the new infected and the new baseball bat melee weapon from the second. Oh, and you're also going to need to deal with a number of known bugs and issues...