It is always best to review your design before sending into fabrication. That is why we have compiled a list of a few free Gerber file viewers:
DFM Now! PCB designers can analyze their Gerber files for fabrication and netlist issues beforehand, to prevent scrap and revision spins. Time is Money... and many PCB Designs can take hundreds of hours to complete from startto-finish. Why not take an additional 20 minutes to thoroughly check your Gerbers with DFM Now! before sending to manufacturing? DFM Now! results in a “Win-Win” situation for both the PCB Designer and Manufacturer. It can be downloaded free of charge at http://www.dfmnow.com/
GC-Prevue provided free of charge, is the industry standard software for viewing and printing electronic manufacturing data. GC-Prevue reads all of the common CAD generated electronic manufacturing outputs, including Gerber-X (RS-274X), Gerber-D (RS-274D), DPF (Barco), Excellon, Sieb & Meyer, HPGL, and HPGL2.
http://www.graphicode.com/
GerbMagic is a free Gerber and Protel PCB viewer. It supports Gerber RS-274x and Protel ASCII PCB file formats (from Protel 1.57 to Protel 2.8) . An extremely accurate vector converter from Gerber RS-274x and Protel ASCII PCB to Adobe PDF and PostScript format.
http://www.bronzware.com/GerbMagic/Download.htm
ViewMate is a free Windows(R)- based Gerber viewer. With ViewMate you can open and view Gerber, ODB++, DPF, HPGL and other PCB file formats. You can view, measure features, and even print your files. Since ViewMate is intended to show the capabilities of the PentaLogix CAM products, ViewMate does not have the ability to save or perform DRC functions. Must register before downloading.
http://www.pentalogix.com/Products/ViewMate/register.cfm
Gerbv is a viewer for Gerber RS-274X files, Excellon drill files, and CSV pick-and-place files and is a free / open source software.
http://gerbv.sourceforge.net/