Here's a nice gallery of architectural elements from England featuring The Green Man.
The Green Man is a common motif in European architecture -- he's widely believed to be a pagan figure dating from antiquity, said to represent the fertility of the soil.
Many have noted the similarities between the Green Man and the Egyptian resurrection god Osiris. (Plenty of others have extended the comparisons to our own Judeo-Christian resurrection god Jesus, but that'll be a whole 'nother posting.)
It's a distinct possibility that the Man is Osiris himself. Persecuted underground by the Roman empire and the early Christian Church, Osiris-worship has ever since taken the form of architectural homage, good-luck gravings, and sprightly garden-gnomes.
The Green Man is just one of those small, rare things that makes our world that much richer.