The European garden spider (Araneus diadematus), also known as the diadem spider, or cross spider, is a very common and well-known orb-weaver spider in Europe and parts of North America.
Araneus diadematus spiders are variable in colour, ranging from extremely light yellow to very dark grey, but all have mottled markings across the back with five or more large white dots forming a cross. The white dots result from cells that are filled with guanine, which is a byproduct of protein metabolism.
The male of Araneus diadematus is usually around 8mm in body length, while the female is around 12mm, though sizes can vary quite a bit. It can usually be found in gardens, woods, and heathlands, and the adults can be seen from late summer until winter.
Description adapted from Wikipedia: http://en.wikipedia.org/wiki/Araneus_diadematus
Top and right of image cropped off in ACR