Update: Left 4 Dead is currently discounted on Steam's website at 40% off (from $39.99 to $23.99), but just for this weekend only.
If you haven't tried playing Left 4 Dead yet because of the price, or aren't sure about the game in general, that's OK. Valve has decided to let you try it out for free.
At 5:01 p.m. PDT Thursday, you'll be able to download and play the game for free via Steam for a whole 24 hours. Better still, to get your full 24-hour allotment of play time Steam allows you to preload it to your account right now.
This isn't a free-forever offer, nor is there going to be a discount available afterward (as with what happened with UT3 in March). Alas, the game will be locked again the following night, but at least it will give you a chance to play and decide for yourself. Obviously, Valve is hoping you'll love it and send some cash their way to keep playing.