Samhain is an open source file integrity and host-based intrusion
detection system for Linux and Unix. It can run as a daemon process,
and thus can remember file changes - contrary to a tool that runs
from cron, if a file is modified you will get only one report, while
subsequent checks of that file will ignore the modification as it is
already reported (unless the file is modified again).

Samhain can optionally be used as client/server system to provide
centralized monitoring for multiple host. Logging to a (MySQL or
PostgreSQL) database is supported.