This inch-long behemoth is a juvenile yellow garden spider (Argiope aurantia). When it is grown, it will be a brilliant yellow and black, and almost three inches across including the legs. (Assuming it’s a female, which is a good guess, as even now she’s bigger than an adult male; the boys top out at about 1cm in body length as adults.)
It is sitting on a weird, messy white splat of web called a stabilimentum, which is distinctive for these spiders. (It’s also the source of the common name “zipper spider”.) The spider can hide on or behind this structure, and may use it for stability when it rocks the web back and forth, which it may do in response to being disturbed or in an effort to entangle an insect caught in the web. In the evenings the spider eats the stabilimentum and replaces it with a new one.
The spider above left is maybe an inch and a half long including legs; the spider above right is 4mm across! It was so tiny I could not see it at all until it moved — but it’s sitting on that distinctive stabilimentum already. That individual zipped back and forth from one side of the web to the other while I tried to take its photo, hiding behind the stabilimentum.
Yellow garden spiders are nonaggressive, and will only bite if provoked; the bite is said to be like a bee sting. They are generally sedentary, waiting in their webs for prey to come by, and they eat annoying insects like mosquitoes and flies, so I’m always happy to have them around.
In this photo you can see the spider’s most recent molt still hanging in the web!