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Steve, Have a look at ContourCam magicsystems.com

Presuming you're after 2.5D rather than 3D.

Alibre does it as a series of parallel cuts, moving the tool vertically as it goes. Good for making pretties but I prefer something that will follow an...

Cut2D exports to Mach fine, although I have a few cad / cam programs I use cut2D for a lot of simple stuff like panel cutouts because it's so quick.

As regards 3D programs, there isn't an easy one...