One thing Garry's mod has spawned is a plethora of fan-comics made by posing ragdolls in various environments. Arguable the best is
Concerned, named for the writer of a letter to Dr. Breen read during the beginning of the game. You'd had to have played the game at least once to get some of the jokes, and if not, the creator has notes for most of the comics that explain things, 'cause he's a junky for commentary. The comic centers around a guy named Gordon Frohman, who arrives in City 17 a week before the other Gordon does. Unlike Freeman, Frohman likes City 17 a lot, and is a big fan of Dr. Breen. The tie-ins to the game are what make the comic so much fun to read, so if you haven't played the game, the comic probably won't appeal to you.
Anywho, the most I did with Garry's mod was to create massive destructive machines, including a robotic arm powered by rockets, and a HUGE carrousel of death (basically a big column with shipping containers attached to it by ropes, with rockets on the containers to make them shoot around the column). If you own HL2, get Garry's mod. Simple as that. At the very least, you'll have some fun screwing around with the facial editor.
EDIT: And Choc, it wasn't meant to be an SDK. It's merely a sandbox mod for spawning and editing the various entities in the game engine. Of course, merely is a relative term, as the things you can do with Garry's mod are pretty awesome.