Nagios
Official site
http://www.nagios.org/
Comments:
Open source :-)
Start-ups using:
Skimlinks
Soup.io
SMS notifications using
any-sms
(german provider)
Plug in SEO
Installed within an hour using pre-built VM at
http://www.trickytools.com/php/metanag.php
Works well monitoring Windows too, and smaller footprint than Microsoft System Center