As powerful and useful as Tripwire Open Source is, it’s also complex and CPU-intensive. Furthermore, if you run “commercial” operating systems such as Windows or Solaris, no free version is available. Therefore, two 100% free and open source alternatives to Tripwire are worth mentioning.
The Advanced Intrusion Detection Environment (AIDE) is designed to meet and exceed Tripwire’s functionality and is available from http://www.cs.tut.fi/~rammer/aide.html. As of this writing its version number is 0.8, which reflects its youth: this may or may not have performance and stability implications. (For what it’s worth, based on recent postings to the AIDE mailing list, AIDE seems to have more compile-time than runtime issues.) AIDE is 100% free to run on any of its supported platforms, whether in commercial or noncommercial settings.