Due to a bug in Gamestop's system, you can receive a demo code without having pre-ordered the game. While I plan on buying the game and have already reserved a copy, I know some people would like to play a portion before they put down money - and thus I have decided to post this information. As far as I know this is perfectly legal but if the moderators feel otherwise they are free to delete the topic.
To obtain a free Left 4 Dead demo invite code follow these steps:
1. Visit GameStop