Is Steam power the future?

Garry's Mod
Not a game as such, but vital for H-L2 owners. This constantly evolving work lets you spawn any object or character from H-L2 or Counterstrike, then weld, pose, set fire to, colour, magnetise and add balloons and wheels (and a great deal more). Other peoples' creations can be downloaded, too. It's hilarious.

Above: This, a floating bed or a fully-functioning, five-storey robot on fire - anything is possible

Above: This, a floating bed or a fully-functioning, five-storey robot on fire - anything is possible