What kind of spider is this? It was found in Nova Scotia, Canada and is about the size of a quarter.
Orb-weaving spiders are quite common, and can be found nearly anywhere in the world, excluding Greenland. The following is a distribution map for the species A. diadematus, of which, as you can see, is mostly contained to North America & Canada.
Dark gray: present; Light gray: no record. (source)