Nagios has been a standard-bearer for a long time, being developed originally by Ethan Galstad and included in Debian and Ubuntu for quite some time. In 2007, Ethan created a company built around providing enhancements to Nagios called Nagios Enterprises. However, for several years now there have been competitors to the original Nagios.
The first to come along was Icinga. This was a direct fork of the Nagios code that happened in May of 2009; the story of what lead to the fork was admirably reported by Free Software Magazine in April of 2012. In short, many developers were unhappy with the way that Nagios was being developed and with what they perceived as its many shortcomings which Ethan could not or would not fix. From Ethan’s standpoint, it was more about the enforcement of the Nagios trademark. The article summed it up best at the end: it’s complicated.
The H-Online also had an interview with Ethan Galstad about the future of Nagios and some of the history of the project.
Icinga is now in Ubuntu Universe and has been since Natty. It is also available for Debian Squeeze (current stable release).
Another project is Shinken: rather than a fork, it is a compatible replacement for the core Nagios code. When the Python-based Shinken code was rejected (vigorously) in summer of 2010 as a possible Nagios 4, it became an independent project. This project is newer than Icinga, but shows serious promise. It too, is now available in Ubuntu Universe and in Debian Wheezy (current testing release).
It is unfortunate that such animosity seems to swirl about Nagios; however, Icinga and Shinken appear to be quite healthy projects that provide much needed enhancements to Nagios users – and both are available in Ubuntu Precise Pangolin, the most recent Ubuntu LTS release.
I don’t know if Icinga or Shinken still work with Nagios mobile applications. If it’s just the URL, then the web server could rewrite the URL; if there is no compatible page for the mobile applications, then they can’t be used. However, I’d be surprised if there was no way to get the mobile apps working.
I’m going to try running Shinken and/or Nagios on an installation somewhere; we’ll see how it goes. I’ll report my experiences at a later date.
UNIX Administratosphere 
Icinga has its own mobile app for Android an iOS
https://www.icinga.org/about/icinga-mobile/
But it seems that Icinga also works with aNag https://wiki.icinga.org/display/howtos/aNag
Shinken supports Livestatus API based Mobile interfaces and can run a native Shinken mobile
A typical mobile interface will not be aware of Shinken Business Impact or visualizing Business Process relationships but it will provide the standard expected Nagios-like output.
Note that from a security perspective all the interactive or bidirectional mobile interfaces are bad ideas…
If testing a new installation today, I would suggest using the git version. (Testing) As this version is destined to be 1.2, it has a great many performance, stability and feature improvements. It also has been through professional testing. Community and commercial players have provided some muscle in getting Shinken ready for deployments.
Let the Shinken community know how your testing turns out.