This is argiope aurantia the spider commonly known as the black and yellow garden spider, writing spider, or corn spider.
Hey, cool. A writing spider.
More from Wikipedia, regarding the heavier bit of web in the middle:
The web of the yellow garden spider is distinctive: a circular shape up to 2 feet (60 cm) in diameter, with a dense zigzag of silk, known as a stabilimentum, in the center. The purpose of the stabilimentum is disputed. It is possible that it acts as camouflage for the spider lurking in the web’s center, but it may also attract insect prey, or even warn birds of the presence of the otherwise difficult-to-see web. Only those spiders that are active during the day construct stabilimenta in their webs.