The Steam deal this weekend is a good one for co-op fans. Call of Duty World at War is on sale for 50% off the regular price, and will just set you back $24.99 until May 18th. Adding to the value, the first map pack is included, featuring the co-op zombie slaying mode "Verruckt".
If you haven't got this title yet, and you're a PC co-op fan, it may be quite a while before you see a deal this good. Don't forget to join the Co-Optimus group on Steam, either. Snap to it, soldier!