I also recommend GC prevue. The old version, its much more easy to use than the later ones. You can download it here:
http://www.pcb-pool.com/ppuk/service_downloads.html
Instructions:
http://www.pcb-pool.com/ppuk/service_downloads_gerber.html
I successfully used the old version of GC Prevue - but couldn't get the new version to work properly (the old version ceases to work after a certain time).
So I tried Viewmate from:
http://www.pentalogix.com
This is also free and seems to work very well......