Gerber viewers:

We strongly recommend the use of a stand alone Gerber viewer. What you see is not always what you get. Don't assume that what you see in your design software is what the manufacturer will see. There are sometimes wide variations between what the design software shows and the output in the Gerber files. It's wise to use a third-party review to check your design. You will find that files output in 274-x format will import into most of these packages with no additional configuration needed.

The following are link is to a stand alone Gerber viewer vendor www.pentalogix.com