Agapostemon has at least 45 species in the genus. This label covers them all -- thus the "?" in the scientific name -- from Agapostemon angelicus to Agapostemon virescens.

Apparently these things are common in the Western Hemisphere, from Canada to Argentina, though I just saw my first one today, in Phoenix, AZ. They are generally green or blue -- head and thorax always, sometimes also the abdomen in some females. They superficially resemble various members of the tribe Augochlorini, which are typically metallic greenish in appearance.

Sources:
http://bugguide.net/node/view/7997
http://en.wikipedia.org/wiki/Agapostemon