I played around with Garry's mod for an hour or so just messing around like attaching thrusters to barrels to make crude exploding rockets and other silly nonsense. It has a lot of potential for interesting world interactions, I'm just not sure I have the patience for some of the more intricate setups. It was on sale for $2.50 and I easily got that much entertainment out of it in an hour though.

Is Garry's mod any good? I have it but have never tried it long enough to figure it out.

One of these days we should do some multiplayer game together! Buzz me if you're online...