Also a term used to describe a secondary action or occupation, particularly an odd one. For example, my father (a judge by trade) was known at his office for moonlighting as a Truly Nolen man, because he does all his own pest control (out here, that means pack rats) and helped several other judges and staff through their own rat problems.

And Everything2 is officially an open database but moonlights as a self-help clinic. Or something like that.