The "perfect circle" cutter for a router, made from scrap 1/4" ply and a nail. Used it to install some deckplates this weekend.
The nail is banged through the plywood, measured from the outside of the router bit to the nail at a distance equaling the radius of the circle wanted. A small pilot hole is drilled in the center of the area to be cut, and the bit is either plunged or a larger hole is drilled on the periphery of the circle to be cut and the bit is plunged through that hole.
This is an old trick I'm sure, but makes perfect circles every time.