Jérôme Vuarand developed an application to visualize Gerber data, a simple 3D viewer for Gerber files. This tool helps to review Gerber files before sending them to a PCB fab house:
Over the course of the last 5 months I developped an application to visualize Gerber data. The goal was primarily to review Gerber files before sending them to a PCB fabrication house. It is very similar to the Mayhew Labs online Gerber viewer, except it’s offline (and has a few different features).
My tool is program for Windows (32bits or 64bits) that use OpenGL for visualization (you will need a decent/recent video card). You can simply drag and drop Gerber files on it, or use the command line to configure it more deeply. I set up a page explaining most of that and giving download links