It is of primary importance the visualization of a PCB before to send it to the printed circuit houses to realize even a small production. The format required is primarily the Gerber format (R274X), sometimes using the files extensions slightly modified by the maker.
In any case before to create a real production it is a must to check all the tracks, pads and wires just to avoid the wasting of time and money.
In the freeware world there are a couple of programs very useful to achieve the result: Gerbv (for Linux) and Gcprevue for windows (owned by DesignSpark; a free account is required):
Gerbv is simply perfect for such a job.
Link: http://gerbv.geda-project.org/
There are also two or three online viewers particularly indicated to see and print RS274x files. This web site are very popular to get a quick image of what it is going to realize without install anything on our pc:
