Monit - monitor processes/services/files/directories/etc. and react if necessary (for Linux/Unix)

"Monit is a free open source utility for managing and monitoring, processes, programs, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations."

To install Monit (and optionally Munin) on a Debian Squeeze server, you can check out this tutorial at howtoforge.com.