Gasteracantha cancriformis (Spinybacked Orbweaver) ♀.
Size: approx. 8 mm. Photo: 08/28/11, noon.

Other common names of Gasteracantha cancriformis include Crab Spider, Jewel Spider, and Smiley Face Spider. The six characteristic (spine-like) projections are often red and typically larger than in this specimen. In Florida specimens, the dorsum is white with black spots; elsewhere the dorsum can also have other colors. They often live in or around trees and are most common in Fall. Their lifespan is short and ends with reproduction. 
bug guide (this photo):