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.

gerbv also supports drill files. The format supported are known under
names as NC-drill or Excellon. The format is a bit undefined and different
EDA-vendors implement it different. But basically you need to have the
tools definition in the file, then the parser is quite tolerant. The
different holes are shown as dots in the (scaled) correct size.

The different layers of the PCB are separated into different files. gerbv
can load all files at the same time and display them "on top of each
other". You can independently turn them on and off.

WWW: http://gerbv.geda-project.org/