Wikipedia: Domestic AC Power Plugs and Sockets
NEMA Connectors (Wikipedia entry)
(NOTE: the above Wikipedia page is wrong about the vertical orientation of receptacles. The ground pin goes on top. See here for Wikipedia relevant discussion entry, and consult the ANSI/NEMA WD 6-2002 standard for the final word.)
The ANSI/NEMA WD 6-2002 standard: (local copy of PDF file), (original NEMA download page)