Similar to last recommendation for Killing Floor 1. Lots of zombies to slay, a floor to kill, and fun varied classes for 4+ people.

Up to 16 player multiplayer games - pick a civilization and journey through the ages, researching tech, waging warfare, and getting resources. War hype.

Lots of zombies - loads of money. Fun game to play with a group of 4-ish people.

Horde modes and fun multiplayer modes. Plus it's free to play, pretty easy to run, and has a lot of hats.

Left 4 Dead 2 is a really fun game to play with friends, either the single-player aspect or Versus Mode, which is up to 8 players.