pakemon, a.k.a. Packet Monster, is an Open Source Intrusion Detection System. It performs in a similar fashion to snort by monitoring all network traffic, searching for specific data patterns and outputting logs of matched traffic. It is highly configurable and is able to use ArachNIDS signature files that can be found at
http://www.whitehats.com/ids/index.html
pakemon is available at
http://www.inas.mag.keio.ac.jp/ids/pakemon/index.html