Last week I've attended LSPE meetup,
Topic - Monitoring
I was really surprised that a lot of people don't like Nagios, but still using it and in many cases alternatives were not even considered ...
For the ones who'd like to check other solutions - I'd recommend to start with
Xymon and OpenNMS
Xymon - Very fast deployment, agents available for any UNIX platform and win32. Scalable to thousands hosts. Easy to write or customize test scripts.
OpenNMS Perfect solution for network/SNMP monitoring. Definitely needs more tuning for advanced options or for testing non-standard services.
Both could be deployed within the same environment and efficiently complement each other.