Garrysmod is a game based around the various props used in many of valve's games where the players can manipulate them any way they choose. With easy to understand mechanics and instructions you could be making your own fan videos or map simulations within minutes.
CPU: Pentium 4 1.8GHz and Athlon XP 1600+ or better
Video Card: DirectX 8.1 level Graphics Card (Requires support for SSE) or better
Free Disk Space: 150MB