a sandbox based off of the half life 2 series. it allows you to take items and weapons from the source games (made by valve) and manipulate them anyway you wish, with ropes pulleys welds ramps bath-tubs wheels thrusters etc etc etc, and a limitless number of addons and a wonderfull implementation of the lua scripting language that allows any ol' pc user to script their own spawnable/usable items. pretty much once you've played the half life series, no matter what games you play, you will get hooked on gmod and its endless fun, specially since it only costs 10 bucks.
which is why Garry Newman will never release gmod in retail, as soon as he would, it would just lead to more pirating, (yes, steam games can be pirated, but it takes some effort) first time i got gmod, i got it with CS:S, this Christmas im getting the orange box so i can have all the half life/portal/tf2 content loaded into gmod. (my friend lent me his steam acc and i played the half life series on his).
and for all you steam pirates, FU, its an insult to Garry to pirate his game, especially since it costs so little and the fact that Garry updates gmod almost monthly. (pirated games cannot be updated)