Amazon has the best deal (when you factor in shipping) on the Dremel 678-01 Circle Cutter, $10.80 w/ SS shipping.
The Dremel 678-01 Circle Cutter makes circles from 3/4″ to 12″. You drill/poke a small hole in the center of the workpiece and insert the pin center of the jig in it, and adjust the Dremel holder to get the correct diameter. It’s a good basic circle routing jig for the Dremel. It also has a straight edge guide that clamps to the tool, allowing you to follow an edge for straight cutting. Granted, for most jobs you want a router, but given that the Dremel now has cutout-tool like functions and cutters this is a good way of adding functionality to the simple Dremel tool.