Today only Killing Floor is on sale on Steam for 75% off - that's only $4.99 in the US (other territories will be 75% of the normal price as well). So if you don't have Killing Floor, or have friends that want to get Killing Floor, or want to buy Killing Floor for someone as a gift, now is the time to act!!! Head on over to the Steam store page now:
Killing Floor Steam Store Page