Gary's mod is a tiny little program that you can buy on Steam (a game server client) that allows you to use content from other games (such as half life and team fortress 2) and do whatever you want with it.
You can create things, destroy things, or pose characters... which I like to do.
Here's the latest thingy I made.
go here for a larger view.
http://redlion.deviantart.com/art/Swimmin-Hole-109391139