What's the difference between Icinga and Nagios?

Icinga is unique as a monitoring system and an ambitious project. This can be seen in the major differences to Nagios in Icinga’s architecture, two web interfaces and reporting capabilities, features for easier addon development, as well as its open and transparent development style.

Icinga 2: Concurrent to improving Icinga as a fork of Nagios, the Icinga Project is also actively developing Icinga 2. There are two development branches: Icinga 1 (Nagios fork) and Icinga 2 (Core framework replacement / Nagios rewrite).