It's called powerline networking. As far as I know, the adapters (not ethernet adapters) are PCI cards, though I don't know if there are any with Mac drivers. Cool? Sort of, though speeds aren't anything to crow about. Also, speed and reliability depend an awful lot on the quality of your house electrical wiring. If Uncle Harvey added a ceiling fan for you a few years back, you could be in for a bad surprise.
Anyway, a search for "powerline network" should get you started. I think Linksys makes powerline products, but it hasn't really grabbed my attention, so I haven't paid much attention to the subject.