Windfire Circle Tool
The circle tool by Tim Elverston of WindFire designs is a thing of great beauty. A progressive set of circles in stainless steel, with sizes that work perfectly together. Use the tool with a pencil, a hot knife or a scalpel. a great deal of thought has gone into the design. Read about them on the Windfire site
Also watch this video by Adam Savage, where he explains that he uses the circle tool every week.