Well, while it doesn't have a true definition of a game, it's very popular for those who love to mess around with props, and several youtube movies are often made in Garry's Mod (the infamous Louis and his pills, for example)
It's pretty much a sandbox which lets you do whatever you want. One can download maps, props, vehicles, ragdolls, non-player characters, weapons, etc and customize to their needs
They even have maps and props which allow you to build a fully working train set.
Anyone here plays this game?