The Naemon core is our version of the Nagios 4 core, which has a lot of extra bugfixes - notably, NASA can use Naemon to monitor their supercomputer cluster without any special configuration tweaking. For the time being, it should function as a drop-in replacement for Nagios, but with additional work spent on improving performance and stability. In the future, compatibility will most likely diverge, but we will make sure that external API's behave the more or less the same, such as object configuration, livestatus, the query handler, the object cache file and status data file.
The Naemon project also includes a GUI, but not the old Nagios CGI interface - instead we have a fast, modern and more useful replacement, Thruk.
You can test the online demo of Thruk (the web gui of Naemon) at http://demo.thruk.org
Binary packages for redhat, sles, debian and ubuntu are available at labs.consol.de as well as in the testing Labs Repository.