Do you know about samhain http://www.la-samhna.de/samhain/
I will install it instead of tripwire.
I have no idea. I've heard of it, but never used it. IDS are a cool idea, but can be a bit time-consuming to manage. I've used TripWire quite a bit in the past, but don't have it on any current systems. Let us know what you think of Samhain! (Some others probably have opinions, too, of course.)
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Isn't this kind of software intended for large(r) deployments, like 100+ servers?
In any case, whether or not deploy IDS apps of any kind, a log analyser is important. Logs are full of noise and clutter and hard to read and time consuming.
http://www.loganalysis.org/ has listed a bunch of tools to make that task easier. You may want to dig into that. If you deploy Samhain, you may want ACID too.