Garry’s Mod is a sandbox game where you can literally do anything you can think of. You can make funny scenes using ragdolls, the tool gun, and the physics gun. You can use anything from valve made characters to community submitted items, including maps and game modes. Two of my favorite game modes are Prop Hunt and Trouble in Terrorist Town.
Trouble in Terrorist Town (TTT for short)
In Prop Hunt, there are two teams: the seekers and the props. the props can pick up almost anything and turn into it, so they can blend in. the seekers have to find and kill the props with an arsenal of guns. For TTT, there is a group of innocent terrorists, but every 1 in 4 is a Traitor. Traitors have to kill all the innocents and have access to a point shop, where they spend credits earned with each kill to buy traitor only gear, from C4 to silenced assault rifles to armor. There are many other game modes, so you should check this game out.
Prop Hunt in a nutshell
Available on Steam, PC only.