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