Bronzware
GerbMagic, is a fast and easy to use, free Gerber viewer. Input formats include; Gerber RS-274x, and Protel v2.5 through v2.8 This software can convert and export to PostScript, PDF, TIFF, BMP and RID, also you can plot with this software. Check out the website for more information.

Roneda PCB Design Consultancy
Viewplot, is the CAD Viewer, Translator and Editor all in one. It's input formats include the following; Gerber RS 274-D, RS-274x (Extended Gerber) HPGL, AutoCAD DXF, Excellon Drill, Bitmap (monochrome), and Tiff (monochrome). The export formats are; Gerber D & X, HPGL, AutoCAD DXF, Excellon, Bitmap, Viewplot Job format, Do_file (Ample for MGC Bs), and PDF. This system can interface with Mentor Graphics Board-Station. For more information or the download the free viewer check out their web site.

Wise Software Solutions Inc.
GerbTool & VisualCAM, is a suite of PCB CAM tools that offer full analysis that includes a powerful Design Rule Check (DRC) system, and Design for Manufacture (DFM) analysis. This system can import Gerber 274, & Gerber 274x, Fire 9XXX, Barco DPF, DXF (Auto CAD), HPGL & HPGL-2, ODB++, IPC-2581, CAM350, PADS ASCII, Excellon v1 & v2, Sieb & Meyer, and IPC-D-356 & 356A. The export options include Gerber 274-D & 274-X, Excellon, IPC-D-350, PostScript, and Bitmap. Check out their web site for more information and to download the trial Software.