Free Gerber Viewer
Gerber Viewer (gerbv) is a viewer for Gerber files. Gerber files are generated from PCB CAD system and sent to PCB manufacturers as basis for the manufacturing process. The standard supported by gerbv is RS-274X. The basic difference between RS-274D (the old standard) and RS-274X is basically the addition of apertures in RS-274X. It might be possible to make an RS-274X file out of an RS-274D file and an aperture list.