Open Source Tripwire is a free software security and data integrity tool useful for monitoring and alerting on specific file change(s) on a range of systems. The project is based on code originally contributed by Tripwire, Inc. in 2000.
Open Source Tripwire functions as a host-based intrusion detection system. Rather than attempting to detect intrusions at the network interface level (as in network intrusion detection systems), Open Source Tripwire detects changes to file system objects.
When first initialized, Open Source Tripwire scans the file system as directed by the administrator and stores information on each file scanned in a database. At a later date the same files are scanned and the results compared against the stored values in the database. Changes are reported to the user. Cryptographic hashes are employed to detect changes in a file without storing the entire contents of the file in the database.
While useful for detecting intrusions after the event, it can also serve many other purposes, such as integrity assurance, change management, and policy compliance.
Personally, I use Tripwire in a simple CentOS server, and have configured it to automatically send me and email after a period of time, and it works just great!
You can download Tripwire from here