Patent leather beetle
The handsome beetle above goes by a variety of names. Some people call it a Patent Leather Beetle, some call it a Bessbug, and still others know it as a Betsy Beetle. The scientific name is Odontotaenius disjunctus.
Betsy beetles munch away on rotting wood, digesting the little microbes within it. Several beetles may live together in a colony of sorts. They communicate with one another by "talking." If you pick one up (don't worry, they seldom bite) and hold it near your ear, you can hear the beetle speak. It is actually called stridulation and results when the beetle rubs its wing against its abdomen. The larva, or grub, can talk as well -- presumably communicating with the adult that cares for it.
One note about looking for things under and in logs: be careful where you put your fingers and ALWAYS return the log to its original position. Decaying wood, and the area under it, is home to a wide variety of organisms and some of these little micro-habitats have taken years to develop. So explore, but do it responsibly.