Network Flight Recorder - commercial IDS by Marcus Ranum & Co.

Libnids - a library supporting TCP reassembly, port scan detection, and other goodies.

Nidsbench - a network intrusion detection system test suite designed to provide better tools for evaluating NIDS products and to help standardize a testing methodology for the purpose of objective comparison Packet Monster - was developped to develop shared IDS components based on the open source development process model. The current version of Packet Monster monitors all TCP packets on your network and captures assigned string patterns for specific socket ports. It generate a detection log that includes date, time, detected signature and the entire log of the detected session. This is still a preliminary implimentation so there are many things to improve.

Snort - a one-of-a-kind content-based IDS tool that generates alerts to syslog and/or text files. Comes with nearly 50 attack signatures.

SHADOW - tcpdump-based Traffic Analysis IDS tool by Naval Surface Warfare Center