Uber has released Hyperbahn – a service discovery for large services.
Riemann is a new modern open source monitoring tool under development (currently at version 0.2).
Kong is a new open source API gateway built on top of nginx. An API gateway provides a thin layer above an application or a service protecting it by adding: authentication authorization rate limiting
Dashing Jenkins Job is an excellent dashboard for Jenkins that can be customised according to the needs of the development team. It is based on Dashing.
Etsy has a great presentation how to improve and extend Nagios functionality by adding configuration automation and reminders to the team:
Nagios is a great open source monitoring solution. However, it is missing an API to retrieve information and status about the monitored hosts, and perform operations without manually going to the web interface.