Garry's Mod is sort of a First Person shooter, sort of based off of Half-Life 2. It is a mod of tens, hundreds maybe even thousands of existing games.

Goat Simulator is a fun 3d platformer with amazing graphics with many NPCs and Entites to interact with. You can't die in the game. You can only explore

It is a suspenseful, thrilling and fun game with a storyline that consists of the World being taken over by Zombies and you are the only survivor.

It is a wonderful first-person shooter expirience with many additional items to motivate the player to play more and interact with other players.