Pandora FMS is a monitoring software which helps you to detect problems before they happen, managing your IT infraestructure: servers, networking and applications.
Pandora FMS - F is for "Flexibility", if something has insformation, Pandora can grab it.
Nice GUI with Ajax/HTML interface. Nice integrated reports and graphs, and very nice management. Very high scalability (up to 2500 monitored servers with a single monitoring server) and excellent documentation (800 page manual in Spanish, English and Japanese).
23 developers from different countries in eight years. That's Opensource !