GC-PreviewPlus

Provided free of charge, GC-Prevue 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. Since it's original release in 1987, GraphiCode has steadily improved GC-Prevue, thanks to its use by tens of thousands in the PCB industry.

Viewmate

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.