A White-banded Crab Spider
A Yellow Garden Spider
UPDATE: August 15, 2010
Recently several people have clicked through to the larger images of the above photos and have left messages regarding whether one or the other of these spiders are poisonous if they bite.
I'm an amateur photographer. I'm not a biologist, a spiderologist or whatever those who study spiders are called. I most certainly am not a doctor, or member of the medical profession.
The best I can do is refer you here. None of these spiders seem to appear in this article. However, for goodness sakes, if you have been bitten by one of them, call or visit a medical facility immediately.