Nessus is a security scanner. That is, it's a program which will scan a
given network and will seek for vulnerabilities which could be exploited
by some remote intruder. 

The Nessus Project was originally started by Renaud Deraison 
(deraison@worldnet.fr). Many people contributed in many ways to the 
project, and the Nessus core team is now made up of Alexis de Bernis
(alexisb@mygale.org), who is the Java specialist, Noam Rathaus 
(dolittle@isrealmail.com) who is in charge of the Nessus client for
Windows, and Renaud Deraison who is still here and who is the project 
leader. 

Check the homepage at: http://www.nessus.org/