So I recently bought a pair of circle lenses from geoeyecandy.com. I got it in Geo Angel Brown and I LOVE them. They're pretty comfortable once you get used to them even though they are thicker than regular contacts.
The purpose of circle lenses is to give the impression of a larger iris diameter and therefore giving you a "dolly eyed" look. They do this by adding a dark colored ring (usually black) around the outer edge.
As you can see from the picture above, the iris of the eye on the left looks significantly larger than the one on the right.
Warning: For those who are thinking about purchasing circle lenses, it is not recommended for extended wear and by that I mean over 8 hours a day. Since these contacts are thicker, they let less oxygen into the eye.