Went for a jog in the park, and saw an absolutely gorgeous dragonfly, which turned out to actually be a male ebony jewelwing damselfly. It's metallic green with wings that are either black or so dark green they look black. There are pictures here (warning, contains damselfly pr0n):
http://www.fcps.k12.va.us/StratfordLandingES/Ecology/mpages/ebony_jewelwing.htmI guess they're pretty common but I'm not sure I'd seen one before.
I really need to get an Ohio field guide; it would be much more efficient than googling.