Dave Czaplak of Maryland, USA, has some very good pages on Moths and Butterflies, (http://odolep.com/) but the most impressive parts of his site are on odonata--so I wanted to be sure to put a link in that category. (I've put as the main link above his page on damselflies.) He has done some beautiful drawings of damselfly abdomens--very useful in identification.

The pages on odonates are:
ID Page for Mosaic Darners in MD
ID Page for River Cruisers in MD
ID Page for Hanging Clubtails in MD
ID Pages for male Damselflies in MD
ID Page for Powdered and Blue-fronted Dancers