Ossim stands for Open Source Security Information Management. Its goal is to provide a comprehensive compilation of tools which, when working together, grant a network/security administrator with detailed view over each and every aspect of his networks/hosts/physical access devices/server/etc…
Besides getting the best out of well known open source tools, some of which are quickly described below these lines, ossim provides a strong correlation engine, detailed low, mid and high level visualization interfaces as well as reporting and incident managing tools, working on a set of defined assets such as hosts, networks, groups and services.

All this information can be limited by network or sensor in order to provide just the needed information to specific users allowing for a fine grained multi-user security environment. Also, the ability to act as an IPS (Intrusion Prevention System) based on correlated information from virtually any source result in a useful addition to any security professional.

Components

Ossim features the following software components:

• Arpwatch, used for mac anomaly detection.
• P0f, used for passive OS detection and os change analisys.
• Pads, used for service anomaly detection.
• Nessus, used for vulnerability assessment and for cross correlation (IDS vs Security Scanner).
• Snort, the IDS, also used for cross correlation with nessus.
• Spade, the statistical packet anomaly detection engine. Used to gain knowledge about attacks without signature.
• Tcptrack, used for session data information which can grant useful information for attack correlation.
• Ntop, which builds an impressive network information database from which we can get aberrant behaviour anomaly detection.
• Nagios. Being fed from the host asset database it monitors host and service availability information.
• Osiris, a great HIDS.
• OCS-NG, Cross-Platform inventory solution.
• OSSEC, integrity, rootkit, registry detection and more.

From : http://ossim.net/