Nessus is a open source security scanner from Tenable Network Security. NCSU has a nessus server on itd-security-scanner.unity.ncsu.edu port 1241.
This is a free client that connects to the nessus server. There is also a commericial client available at www.nessus.org.
This is the nessus documentation page. The best introduction comes from Security Focus at [part1] [part2] [part3]