The "Nessus" Project aims to provide to the internet community a
free, powerful, up-to-date and easy to use remote security scanner.

A security scanner is a software which will audit remotely a given
network and determine whether bad guys (aka 'crackers') may break
into it, or misuse it in some way.

Unlike many other security scanners, Nessus does not take anything
for granted. That is, it will not consider that a given service is
running on a fixed port - that is, if you run your web server on
port 1234, Nessus will detect it and test its security. It will
not make its security tests regarding the version number of the
remote services, but will really attempt to exploit the vulnerability.

Nessus is very fast, reliable and has a modular architecture that
allows you to fit it to your needs.  Test the security of your
network.