Steam User Rating: 94% Positive
Wishlist It: Steam Store
Okay, so the first two entries on this list may be a stretch, but it wouldn't be the first time an early-access/relatively new game made it to the Steam Summer Sale. Killing Floor 2 is a great co-op shooter made to be played with up to five of your friends.
Fight waves and waves of enemy Zeds as they swarm you and your surviving team-mates. If you're into FPS games, Killing Floor 2 is not something you want to miss this year.