Systraq weekly sends you an email listing the state of your system. Furthermore, if critical files have changed, you'll get an email within a shorter notice. Systraq consists of few very small shell scripts.

It can help you implementing a not too strict security policy. For more demanding systems, you'd better use something like tripwire or tiger. Make sure you really want to do the monitoring this script offers: it might not comply with your site's privacy policy. Getting informed when users' config file change might be too intrusive.

Other similar tools are available in Debian (`diffmon' for instance), but systraq is less intrusive because it can warn for file changes without mailing a complete diff (which is not desirable for e.g. /etc/shadow).