Nagios
http://wiki.centos.org/HowTos/Nagios
http://www.tuxradar.com/content/nagios-made-easy
http://www.thegeekstuff.com/2008/06/how-to-monitor-remote-linux-host-using-nagios-30/
Nagios on Ubuntu
https://help.ubuntu.com/11.10/serverguide/C/nagios.html
NRPE- Nagios Remote Plug-in Executor