What the heck IS this animal? It looks like a hammerhead worm...?

It's a Land Flatworm or Land Planarian - Bipalium kewense, sometimes (colloquially) called the Hammerhead Flatworm, although the head is actually semi-circular with a rounded leading edge. The wikipedia link includes a short clip of a specimen moving along a rock (if your PC supports the playback).

http://en.wikipedia.org/wiki/Bipalium_kewense

nurnord is correct

commonly called a land planarian, can be 1-3 inches long, lives in damp, dark terrain - under logs, rocks, houses, even sometimes on leaves after rain

i had a couple as pets for a while, their awesome