Collectd collects system-level metrics on each machine. It works by loading a list of plugins, and polls data from various sources. The data are sent to different backend (Graphite, Riemann) and can be used to trigger alerts with Nagios.
Sensu was written as a highly configurable, Nagios replacement. Sensu can be described as a "monitoring router", since it connects check scripts across any number of systems with handler scripts run on one or more Sensu servers. It is compatible with existing Nagios checks and additional checks can be written in any language similar to writing Nagios checks. Check scripts can send alert data to one or more handlers for flexible notifications. Sensu provides the server, client, api and dashboard needed to build a complete monitoring system.