This site has links to the homepages of 92 different IDS systems... http://www-rnks.informatik.tu-cottbu...birey/ids.html

And this is a really good FAQ. Way to big to post the whole thing here...
Version 0.8.3, March 21, 2000
This FAQ answers simple questions related to detecting
intruders who attack systems through the network, especially
how such intrusions can be detected.