Host Based Intrusion Detection Systems (HIDS)
Samhain is a file system integrity checker that can be used as a client/server application for centralized monitoring of networked hosts. Databases and configuration files can be stored on the server. Databases, logs, and config files can be signed for tamper resistance. In addition to forwarding reports to the log server via authenticated TCP/IP connections, several other logging facilities (e-mail, console, and syslog) are available.
OSSEC is an Open Source Host-based Intrusion Detection System. It performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response. It runs on most operating systems, including Linux, MacOS, Solaris, HP-UX, AIX and Windows.
Tripwire® software is a security and data integrity tool useful for monitoring and alerting on specific file change(s) on a range of systems. The project is based on code originally contributed by Tripwire, Inc. in 2000
Network Based Intrusion Detection Systems (NIDS)
Snort® is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Combining the benefits of signature, protocol, and anomaly-based inspection, Snort is the most widely deployed IDS/IPS technology worldwide. With millions of downloads and nearly 400,000 registered users, Snort has become the de facto standard for IPS.
Bro is a powerful network analysis framework that is much different from the typical IDS. While focusing on network security monitoring, Bro provides a comprehensive platform for more general network traffic analysis as well.