Host-based Intrusion Detection Systems (HIDS) are a recent development that has been a bit overhyped as late. Don't get me wrong, HIDS are a valuable too. It's just that the technology has been pushed as the solution du-jour a bit more than I care to see. In any case, it should be part of your reptoire for defense-in-depth. Linux Security has an article about a project called Open Source HIDS.