There is Shinken as well (http://www.shinken-monitoring.org/features/ http://www.shinken-monitoring.org/features/) which promises 100% compatibility with Nagios

There is Shinken as well (http://www.shinken-monitoring.org/features/ http://www.shinken-monitoring.org/features/) which promises 100% compatibility with Nagios plugins... plus is written in python (https://github.com/naparuba/shinken https://github.com/naparuba/shinken). Not sure about performance comparisons