Nagios is a monitoring system that originated in the 90s as NetSaint. It is primarily about alerting based on the exit codes of scripts.
Unlike other solutions, the amount and types of data it stores is limited to check state making it suitable only for a very basic monitoring.
Sensu can be considered a more modern version of Nagios. The primary difference is that Sensu clients register themselves, and can determine the checks to run either from a central or local configuration. There is also a client socket permitting arbitrary check results to be pushed into Sensu.
Sensu uses (almost) the same data model as Nagios and shares its limitation of the format it uses to store metrics.