Linux Intrustion Detection?

Woodie asks: "Hi,I'm wondering, after reading Dvorak's article on crackers , whether good intrustion detection software exists for Linux. He specifically mentions a product called "BlackICE" - which I checked out the details of - that sounds very interesting. What Linux alternatives are there? I'm not necessarily expecting an easy to use GUI; some kind background daemon that generates a usable log and that can be preconfigured to respond to certain "attacks" would be great. " How reliable are the results from various Intrusion Detection packages? Are these things worthwhile? Or would do-it-yourself monitors be a better choice?

Update: 11/03 11:58 by C : Jargon was also interested in Linux Intrusion Detection and was curious if there were Linux contenders to the likes of Cybercop Sting, and Mantrap"