This article shows many images of real-life circles that make really neat photos. If you want to see picturesque circles from around the world, this link is a great resource.
This article contemplates the idea of the "perfect circle" in the real world. It suggests that circles formed by nature will have imperfections that are different than the circles that we practice our problems with in school.
This article is a compilation of ideas from both of the previous articles. This Smithsonian Magazine author also suggests that perfect circles do not exist, however, he shows us pictures of how close to perfect these natural circles have become.