This circle-cutting ruler is both more expensive than and more fully-functioned than the cut-a-round ruler, which I also carry.
 
The cut-a-round ruler allows you to cut circles that are an exact number of inches in diameter. When sewn in they end up a half inch smaller, so the finished circles are always on half-inch; a 12" cut circle yields an 11 1/2" finished circle.
 
This Creative Grids ruler allows you to cut circles either an exact number of inches or on the half-inch... it is two circle rulers in one. If you want more complete flexibility in your quarter,  half, and full circle sizes, then this is the better ruler. It allows you to cut:
 
quarter or half circles with extra seam allowance on the straight edges
circles
from 3-1/2" inch to 15-1/2" inch diameter. I use it for both the quarter/half/full circle and the corresponding hole into which it will be set.