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