I noted some discussion about using CutViewer. It is a great program but rather pricey for the average home hobby user.
I discovered CNC Simulator a while back. Although it do not have all the bells and whistles that are in CutViewer, it does do a great job of showing your GCode in action. Works in both 2D and 3D and is a very good training tool.
Highly recommended and it is FREE! That in itself is a plus.
You can down load it at http://www.cncsimulator.com
George
Check out my Experimental Aircraft CNC Cut Parts
http://www.mykitairplane.com