Centreon - Nagios Management Frontend
http://www.centreon.com

Centreon is one of the most flexible and performant monitoring software under GPL license.
Meant for any kind of users, Centreon fits the indicators of systems, networks and applications.
Centreon gathers functionalities that are essential to the monitoring of critical infrastructures.
Being modular, it can evolve depending on your needs thus allowing you to extend its functionalities.