toxicodendron radicans
Did you know that a poison ivy rash doesn't itch when you get it on your face? On the other hand, it itches very much when you get it on your feet and/or legs. At least, such are my observations so far this year. The dreaded weed seems to be more widespread than ever before; clearly the climate this spring suited it. It's too bad about the itching and the terrible blisters, because otherwise poison ivy would make a very attractive (and maintenance-free!) ground cover.
Why have I got it three or four times already this year? I have to defend myself against the charge of simple incompetence: I can recognize the leaves of three as well as anyone, I believe. Rather, the problem is the dog. Specifically, as he is completely immune to p. ivy's effects, he charges through the stuff every which way, and picks up the poisonous residue on his fur and leash. Furthermore, I must note that even when you very strongly suspect you've touched poison ivy at least with your fingertips (where it doesn't seem to have any effect—another point that I should have included above) it is very difficult to refrain from scratching your face, especially when you're being 100% plagued with mosquitoes. Those blisters are healing nicely now, though.