This isn’t exactly a game, but you could play with for hours… if you have interest in sculpting anyways.

Still in alpha stages (meaning it may have lots of bugs, so if you intend to keep your work, save constantly) it’s a very interesting virtual sculpting tool.
In other words, a simplified and intuitive 3D modeling tool.
Created by DrPetter, you can get a copy of the alpha at the official Sculptris website!

Source: TIGSource