Dead Island is currently on sale for $11.99. After the 10% off voucher it’s only $10.80. It uses Steamworks, so you’ll have to activate it on Steam.
Killing Floor is on sale for $5.98. Use the voucher and it’s only $5.39. It also uses Steamworks.
Both are great co-op games IMHO. I think there was an update for KF the other day. There’s a new female model and a bunch of map fixes.
Voucher expires June 6th.