Kicking off the 24-hour Free Trial of Left 4 Dead (which is now underway BTW!), Valve has announced that the popular online zombie shooter is currently on sale through Steam for just $23.99 (a 40% savings). The deal runs from now until Monday, May 4th. So if you dig the free trial and want to add this bad boy to your collection, this should help save you a few bucks! ;)