Monster Mud is probably one of the coolest things ever dreamed of. It's a devilish mixture of Drywall joint compound and latex paint. The end result is a goopy mess that haunted house prop makers just call Monster Mud.
The brilliant thing about Monster Mud is that it can be added to cloth and when allowed to dry it stiffens to a faux stone like finish. A handy thing to have when making statues, tombstones, or anything else that requires such a texture.
Here is a website that details the exact recipe and be sure to check out the prop pages to see what groovy things can be made with Monster Mud.
Also check out this site and this one for more Monster Mud goodies.