http://copperegg.com/whats-a-good-alternative-to-nagios/

Many devops and admins have used, and in some cases continue to use, Nagios as a key part of their cloud monitoring infrastructure. We’ve had many conversations about their Nagios use cases, likes, dislikes, and the relative Nagios strengths and weaknesses. There seems to be a distinct trend in what actual Nagios users think makes a good replacement and alternative to Nagios.