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
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.