I'm looking to install a server monitoring application on my web server (CentOS, LAMP setup). After some reading, it seems that Nagios is the most popular solution, but it was forked to Icinga a year or so ago. Which should I use? Is Nagios still actively developed? Are Nagios plugins compatible with Icinga? Is configuration still a nightmare in Icinga? (this seemed to be the major complaint against Nagios). If you've used both, what are the pros/cons of each?
I would recommend taking a look at Icinga. It's a very active spark of nagios with regular updates. On top of that, it's 100% compatible with nagios-plugins.
The advantage of Icinga is that the make one API available to talk with, instead of a bunch of them. In Nagios you can get info from the Core, Cache, NDOMod, NDO2DB, the MySQL DB, Command line,...
Icinga also has a beautiful and easy-to-use web interface. You can also use NConf with it, which is awesome. Here is a video explaining the differences: http://www.youtube.com/watch?v=jLqhXvGTazI
(It's from Icinga, so it might be a bit biassed)
I use nagios exclusively, and I find configuration to be a piece of cake. I simple create a directory for each "type" of device being monitored, and then a new file for each device. By doing it that way, it's REALLY easy to manage. Plus, I love the ease in writing new plugins... And yes, as far as Nagios goes, it is still in active development (As far as I can tell, they are releasing new versions)...