I picked up today Garry's Mod, a sandbox application of Valve's Source engine. Basically, I get to play with just about anything and everything using the source engine: Half-Life 2, Portal, and Team Fortress 2 (!!!).
There's a moderate learning curve to it, but I'm slowly getting the hang of it. I'm really trying to see what I can do with it, and am pretty proud of my first - albeit not too complex - picture:
Click for a larger image
More to come, for sure.