Cabot
Self-hosted, easily-deployable monitoring and alerts service
... can use data that you're already pushing to Graphite/statsd to generate alerts, rather than implementing and maintaining a whole new system of data collectors. We built Cabot as a Christmas project at Arachnys because we couldn't wrap our heads around Nagios, and nothing else out there seemed to fit our use case. We're open-sourcing it in the hope that others find it useful.