A mod for the Source engine that lets you play around with the physics in multiplayer mod. You can spawn items, stick them together, add forces, etc. Youtube garry's mod for some funny things people have made. It also lets you pose people from HL2 and CS
in different ways, and do funky things with the camera, like blur, etc.
Concerned (a joke on the "Concerned Citizen" from HL2) webcomic was made completely in GMod.