"Nessus is a free security scanner available from
http://www.nessus.org. The project was started and is maintained by
Renaud Deraison. The stable version at the time of this writing is
1.09 and the experimental one is 1.14. The software is released
under GPL and many people contribute to the project, especially for
plugins... while some other people benefit from nessus work without
even mentioning the name (more on this at the end of the article).
Nessus works on many Unix flavors as a client and a server, and on
Win32 as client. Let's have a look at this great
tool."