Nessus is a security scanner that crawls across a network, looking
for well-known vulnerabilities and common misconfiguration.

It has a unique set of features, including automatic SSL discovery,
services recognition (so it will catch, for instance, a FTP server
running on a port different than 21) and its own scripting language.

The Nessus Security Scanner is released under the GNU General Public
Licence and aims to be easy to use while extremely powerful.

WWW: http://www.nessus.org/

PS: To install the lot in one operation, do nessus-plugins first.