The Realeyes IDS captures and analyzes full sessions. When an incident is reported, the graphical user interface will display both halves of the session to determine what occurred. The GUI also provides management of application users, sensors, and a database. Realeyes is a replacement for the RenaissanceCore software.
LibHTP is a security-aware parser for the HTTP protocol and the related bits and pieces. That can mean many things, but the only scenario in which LibHTP has been tested so far is the one when you need to parse a duplex HTTP stream that you have obtained by passively intercepting HTTP traffic. Just feed the raw TCP stream to LibHTP and it will do the rest.
Suricata is an Intrusion Detection and Prevention (IDS/IPS) engine developed by the Open Information Security Foundation and its supporting vendors. The engine is multi-threaded and has native IPv6 support, file extraction capabilities, and many more features. It's capable of loading existing Snort rules and signatures, and supports many frontends through Barnyard2.