Garry's mod is a cool little mod (about 25mb) that let's you, basically, mess around with the half-life 2 physics engine and have loads of fun.
Examples:
Spawn LOADS of items instantly including characters, items, and vehicles
You can play around with characters faces, giving them any expression you want
attach balloons to people and watch them float away, then getting out your shotgun and shooting all the balloons off and watching tham fall to the ground! lol
Attach thrusters to people and watch them whizz around you really fast
Create a giant domino field out of doors
And basically, do whatever you want.
There is just too much to do in the mod it would take ages to write it all down.