Garry's Mod, according to Wikipedia, is "a sandbox physics game using the Source engine." To me, it's much more than that.
Garry's Mod started out as, well... a mod. It wasn't always a stand-alone game and was a total conversion modification for Half-Life 2 until 2006. Due to that very fact customizability and moddability are an integral part of the game itself, which makes the game extremely versatile - you can do just about anything to the game you care to.
You can use it as a machinima film-making tool, a physics simulator, an inventor's workbench, a platform on which to show off your custom-built in-game weapons via the Toybox, and a base on which to make your own custom gamemodes. People have used it to set up complex electronic circuitry, host role-play servers, undergo atomic weapons testing and design their own levels.
If you like customizability, sandbox games, infinite replay value and multiplayer, or need a tool with which to tell your own stories, check this game out.
Originally written August 6, 2012 on Steam