Centreon have their own fork of Nagios (Centreon Engine) [1] and a pretty nice UI (Centreon) that beats the pants off of the crusty old Nagios CGI interface [2]. We've used the Centreon front end (but keeping Nagios as the underlying monitoring engine) for about three years now and have been very happy with it.
What about http://cacti.net/ ?
With Weathermap plugin it is quite good thing for performance monitoring. THold and Monitor plugins are for availability monitoring and alerting. It scales very good with Boost plugin, I had 3000+ hosts, 60000+ datasources with total poll time within one minute.
Many prefer Cacti to Zabbix because the latter is clunky and confusing compared to Cacti's simple straightforward approach with plugins.
But I am actually wondering, how alive is Cacti now, because its releases are constantly postponed, and forum is not as alive as it used to be.
Cacti is just a graphing tool that sits on top of RRDTool . It doesn't provide up/down monitoring and host/service check scheduling, it doesn't provide features such as alert escalation logic, and a whole bunch of other fairly important things that a monitoring engine such as Nagios does.
Zabbix is very good, at least since version 2.0.x which is the one I've used. Once you understand how it works, it's very easy to write agent scripts (plugins) and build templates for it.
It took about 3 months for me to build a monitoring solution from scratch with Zabbix for a little over 100 devices, with just a single vCPU VMware VM for both Zabbix and its MySQL database. Monitored devices included Windows, Linux and AIX machines, but also a couple of large UPSes and a few server rack PDUs.
This was my first contact with Zabbix and during this time I wrote dozens of agent scripts to monitor stuff ranging from DB2 and Microsoft Exchange clusters to physical disks and SAN LUNs.
It's one of a select few software packages that I can say that I really like. It works as expected and it's pretty extensible.
Yep, we moved form Nagios to Zabbix once we crossed the ~300 devices as that is one of the main advantages for Zabbix: it scales pretty nice. Takes a bit to get to know how everything works and set it up correctly but it's a pretty good piece of monitoring software.
naemon is a fork of nagios 4.0 done by the developer that commited 95% of the Nagios codebase: http://naemon.github.io/
His "release" from being a core nagios developer is another interesting story, he talks about it some here (some nsfw language: http://www.youtube.com/watch?v=YgbbyyNIiHc
[1]: http://www.centreon.com/Content-Download/download-centreon-e...
[2]: http://www.centreon.com/Content-Download/donwload-centreon