Let's close this list out with a mod that's so completely bizarre it needs to be played to be believed. Elevator: Source requires Garry's Mod to play, so get that first. This mod isn't exactly a game, more like an experience. You boot it up and you're in an elevator. You go from floor to floor, and every time the doors open they show you a weird, unexpected landscape. Sometimes people get on, sometimes they get off. Awkward conversations sometimes ensue. And you can play it co-op, if being alone in an elevator with total strangers freaks you out.

Download Elevator:Source here.