Can you recommend a free File Integrity Checking Tool?

As financial institutions are pushing my customers to tighten up the security in their infrastructures they are requiring more tools be used to formalize the process. While doing my part to complete a recent assessment survey I found that the vendor is requiring "all computers and servers in [the customer's] credit card environment to use File Integrity Monitoring (FIM) Tools to detect when important files are tampered with".

This particular requirement could be an expensive requirement for a small business to monitor a few dozen computers, especially if server versions are required. However, I know there are a number of free tools available to try, including AIDE, TripWire's SecureCheq, 'another file integrity checker', OSSEC, and others. Are any of you familiar with and willing to vouch for one of these? Or suggest another?