Garry's Mod

http://www.garrysmod.com/
http://store.steampowered.com/app/4000/Garrys_Mod/
http://steamcommunity.com/app/4000

Garry's Mod started out in 2004 as a Half-Life 2 mod where the player could spawn props and attach them to things with rope. Over the years it's grown into a sandbox meme factory filled to the brim with player-generated content with GMod 13 being the crashing crescendo. Even after Garry himself moved on to different endeavors it still goes strong with new Steam Workshop content being uploaded on a daily basis.

And how could we forget all of those iconic machinimas born out of GMod's heyday in the late 2000s? The GMod Idiot Box, Civil Protection, Elliot Goes To School, The Idiots Of Garry's Mod, Full-Life Consequences, War Of The Servers...

 

God, I overloaded my files with 445 mods that every game on my PC started lagging.

 

Was gifted that a few years ago, still love it. Alas, school's letting out and we'll see a bunch of dumb kids who think they're funny / cool, acting like total morons when all you want to do is watch Cinema.