I've had the opportunity to work with Icinga during my internship last year.

Nagios development has stagnated somewhat but Icinga is thriving. It is 100% compatible with all Nagios plugins and most of the configuration tools. Most notably, NConf works very well with Icinga.

They reworked the old Nagios interface and offer entirely new web and mobile interfaces. I must say it's a pleasure to use.