For those of you who don't yet own L4D2, Green Man Gaming is having a deal right now:

http://www.greenmangaming.com/s/us/en/pc/games/action/valve-complete-pack/#b

For 20 bucks (with the additional coupon for 20 percent off) you get all Valve games. Including L4D2. Needless to say I refunded my copy of L4D2 and bought this for the same price.