Any number of irrational and hysterical individuals whose self-indulgent intellectual indolence has led them to a visceral hatred of all things western. Moonbats tend to frequent anti-globalization and numerous other forms of rallies, demonstrations, sit-ins, and the like, tend to look dirty, and can frequently be heard using terms such as, "imperialism". The mark of a true moonbat is the total lack of perspective (i.e. "if a dictator provides free healthcare, I like that dictator" - actual moonbat quote).
Did you hear that George Galloway has praised yet another Arab dictator? What a moonbat!