Re-release of Alien Swarm with now up to 8-player Co-op (4-player still available), more Campaigns, more Weapons, more Extra items and BOTs. The game is completely free, please give it a try!

L4D in the Alien universe. This game is only 3 player Co-op, but is well worth your time and money. It offers a Campaign and lots of progression and Weapons/Classes to level up.

Have not played yet.

L4D/Vermintide set in the Warhammer 40,000 universe. 4 players cooperate against zombies and other enemies in a grim environment. Ranged/Melee Weapons are satisfying and there's lots of progression.

Have not played yet.

Third Person Shooter but has a lot in common with L4D. This is the ultimate bundle to get the first 3 games. It gives a lot of the sniping goodness from the Sniper Elite franchise. Great Co-op fun!

Sequel to the first game. Play as the same 5 Characters, but you can now choose between 4 different Careers for each! It adds more Campaigns and Weapons to the first game! Highly replayable.

Left 4 Dead set in the Warhammer fantasy universe! Play as a Knight, Dwarf, Archer, Wizard or Witch Hunter! The rats have infested the city and you must get rid of this plague!

Deep Rock Galactic is a Co-op game where 4 Dwarves enter missions to find gold and other minerals. The game has a huge progression system and is very replayable due to its randomly generated maps.

Left 4 Dead 2 expands on the idea of the first game. It adds more Campaigns, more Weapons, more Special Infected and in the end that equals to even more replayability! It also supports Steam Workshop.