CamBam is a CAD / CAM application designed to take common CAD files such as DXF and 3DS, manipulate drawings using CamBam's internal geometry engine then produce tool paths and gcode for CNC machines. http://www.cambam.co.uk/

It is a great free tool for visually verifying G-code that has been created by PCB-Gcode. You can find problematic areas such as clearance issues that could result in a combined trace (short). You can use it to aid you in the reduction of sprites, and also methods to reduce your milling time. Brusselsprout has also written a very comprehensive article on PCB Isolation routing: http://www.brusselsprout.org/PCB-Routing/