Bro is a powerful network analysis framework that is much different from the typical IDS you may know.

Key Features

Adaptable

Bro’s domain-specific scripting language enables site-specific monitoring policies.

Efficient

Bro targets high-performance networks and is used operationally at …

http://www.toolswatch.org/2012/08/the-bro-network-security-monitor-v2-1-public-beta-available/