Orchard Orbweaver

I had a very difficult time identifying this spider, but the markings indicate Leucauge venusta, a common garden species. It's a member of the family Tetragnathidae, or Long-jawed Orbweavers. That means their chelicerae are longer than those of other orbweavers. When you come upon a web (always at an angle, with concentric circles), the spider will most likely be hanging upside down in the center, beneath the web, so look for the metallic orange bracket-shaped mark on the underside of its abdomen. The abdomen will probably look white or silverish, with black on the end and some yellow around the bottom edges. Often the abdomen has a veined pattern: black streaks on silver with yellow on the sides. The legs are quite long and thin. A spiderling will have the orange mark but a mottled abdomen, and of course its body will be smaller than an adult's.

Specimen #1:

Specimen #2:

You can learn more about Orchard Orbweavers at:
http://bugguide.net/node/view/2010
http://www.spiderzrule.com/venusta.htm
http://www.floridanature.org/species.asp?species=Leucauge_venusta
http://people.duke.edu/~jspippen/arachnids/leucauge-venusta.htm
http://www.museum.state.il.us/ismdepts/zoology/spiders/gallery.html?RollID=roll02&FrameID=Leucauge_venusta