What are some alternatives?
When comparing ZofzPCB and MCN Gerber Viewer, you can also consider the following products
Gerbv - A Free/Open Source Gerber Viewer
3D Gerber Viewer - FREE Online 3D Gerber viewer
EasyEDA Gerber Viewer - EasyEDA Gerber Viewer is a free viewer for Gerber RS-274X files, Excellon drill files online.