Sometimes the adage "you get what you pay for" is not always true. This certainly applys to computer security tools. Some of the best tools are free and out perform the commercial tools. All the following tools are open source and are free for downloading and use.

Snort

Snort is a excellect intrusion detection system

Portsentry

Portsentry monitors unauthorized network connections to a Unix host.

Nessus

Nessus is a very capable vulnerability scanner.