i've had this 12" one for years and used to use it a lot... now i just draw a line corner-to-corner and use my 1/4" foot to sew on either side.
http://www.amazon.com/Quilters-Rule-.../dp/B000YZAN94
it also comes in 8"
http://www.amazon.com/Quilters-Rule-.../dp/B001701LBY