Black-and-Yellow Garden Spider - Argiope aurantia - probably female (based on size) - at least that is what it seems to be from the three field guides I looked at. I nearly blundered into this web going through a field of wildflowers looking for butterflies. Lots of spiders cannot be reliably classified to the species level in the field, but several Orb Weavers, of which this Argiope is one, can be.