this looks like fun...
do you like, freeze the ragdoll in the air or something?
Pretty much. It's got the physgun code, which was in the original plans for hl2 but was scrapped. You might remember it from the e3 physics demo movie. There's another mod called jbmod that does the old implementation of the physics gun, where you can pick stuff up and move it around and also use the alt fire to weld things together, albeit with some difficulty. Garry's mod foregoes that and switches the crossbow to a welding gun. Shoot one thing, shoot another, they're welded. This lets you create big neat objects out of lots of physics props.
But the physgun has been changed in Garry's mod to allow to you move things around and then freeze them in place with the alt fire. With ragdolls, you can manipulate each individual bit of the skeleton, and then freeze them, allowing you to make all sorts of neat little dioramas. The pics I've got only represent about five minutes of work, with some effort one could create something truly amazing.
By the way, this mod was invented by a chap named (surprise) Garry, a Something Awful forums member. Credit where credit is due! (And you thought SA was only good for photoshops and goatse references)