AIDE(http://sourceforge.net/projects/aide) – Self-described as “AIDE (Advanced Intrusion Detection Environment) is a free replacement for Tripwire. It does the same things as the semi-free Tripwire and more. There are other free replacements available so why build a new one? All the other replacements do not achieve the level of Tripwire. And I wanted a program that would exceed the limitations of Tripwire.”

File System Saint(http://sourceforge.net/projects/fs) – Self-described as, “File System Saint is a lightweight host-based intrusion detection system withprimary focus on speed and ease of use.”

Snort(www.snort.org) – Self-described as “Snort® is an open source network intrusion prevention and detection system utilizing a rule-driven language, which combines the benefits of signature, protocol and anomaly based inspection methods. With millions of downloads to date, Snort is the most widely deployed intrusion detection and prevention technology worldwide and has become the de facto standard for the industry.”