Tripwire

Tripwire is a file integrity checker, a utility that compares a designated set of files against information stored in a previously generated database. Any differences are flagged and logged. When run against system files on a regular basis, any changes in critical system files will be spotted -- and appropriate damage control measures can be taken immediately. With Tripwire, system administrators can conclude with a high degree of certainty that a given set of files remain free of unauthorized modifications if Tripwire reports no changes.

Tripwire has two versions of their product, one that's free and one that is commercial.  The commercial release supports a fast installation and optimized performance as well as keeping critical files in binary format and cryptographically signed, and is Y2K compliant. The free version, though, lacks all of the above, including being resource use intensive.  We downloaded the free version and tested it out.  Our results are here.

We highly recommend checking them out at http://www.tripwiresecurity.com/.

One alternative is the Network Operating System Trinux, which operates on top of an existing OS, such as Linux or Windows, but the problem is that it was written the binaries are compatible with Red Hat Linux 5.2, not Red Hat Linux 6.0.  However, we have heard good things about it, and suggest that you check them out at http://www.trinux.org/.