You can also just make your own circle jig for a router. Its very simple to do. You need a plunge router to use any of these jigs, I suppose it could be done with a fixed base router but odds are you'll probably screw up the cut or hurt yourself trying.
Heres one I made to do my circle cuts because I did not want to wait for my jasper jig to come in.
Basically you just need a thin sheet of something (cabinet toe kick peice here) and take off the stock router base, drill the necessary holes for the mounting screews and hole for the bit and screw it to the router just as you would the base. Then calculate half the diameter of the hole you want to cut, and measure from the far side of the cutting bit that distance out on the peice and then put a screw or nail or something to hold it in place. It doesnt matter if you get the hole for the nail directly centered, so long as its half the distance you want away.
And of course you can test it by just cutting out the smallest hair of material and measure to make sure you got it right.
