David, like Sonya says, you've found a lovely specimen of Araneus marmosa, the Marbled Orbweaver. Charlotte of Charotte's Web fame was a related species.

It's hard to tell whether it's a he or a she. Looks like a girl but usually it's guys you see walking around, looking for a cute Spiderette to date.

If it's a girl, she'd normally be hanging out near a fair-sized orb web, likely off to the side, often hidden but connected by "signal lines" that relay vibrations from caught and struggling prey. Interestingly, these spiders usually take apart (eating the silk), and re-weave their webs each night. Seems like a lot of work but then with their crappy eyesight they can't watch TV.