I added PNP4Nagios to my environment over the last few days. Other than making some simple configuration changes to account for the RPM package-based installation, it installed easily and works very well. It even includes a mobile web interface!
I'm a new Nagios user, and while the standard UI works fine on an iPhone, there is a fair bit of pinching and zooming going on. This UI is perfect for the quick look, and I know I can still pop over to the full UI if needed. Nice looking, and easy to install.