The 'Nessus' Project was started in early 1998, and first released in
April 1998. At this time, the most complete free security scanner was
SATAN, which is clearly outdated, and you could see the emergence of
several commercial ones, that were clearly too expensive.

The Nessus Security Scanner is not only another security auditing tool. It
is a security auditing as I think it should be - never trust the version
number, never trust that a given service is listenning on the good port
(do all the web servers on earth listen on port 80 ?).

The Nessus Security Scanner is free, open-sourced and wants to be easy to
use. 

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

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