Hi folks:
I have been using Secunia for Vulnerability Scanning for some time now.
There are two free products:
The Secunia Online Software Inspector (OSI) - http://secunia.com/vulnerability_scanning/online/
The Secunia Personal Software Inspector (PSI) - http://secunia.com/vulnerability_scanning/personal/
Note that the PSI is downloadable and complements NAV/NIS.
From my experience, the PSI works best when configured not to start-up at boot time - use it only on-demand.
These softwares look for problems with programs that have Security Holes in them and give you a method in which they could be resolved. Both have helped me on many occasions with a preference for the PSI, as it is more thorough and customizable.
Please evaluate for yourself!
Cheers.