PC Games?

Garry's Mod

I feel like now this should be a Steam essential since it's one of the most popular games with an active community. There is almost 1 million addons on the workshop and while some of these are stupid a lot are very high quality and really fun to try. I think what makes the game so good is the multiplayer considering that's where you'll be 99% of the time (I've only played in single player once for the achievement). With lots of interesting gamemodes to try I don't think you'll find this game boring