A fork of the System Status module, but with a Symfony app for you to install to act as a server, rather than using a paid third-party service. Monitor core/module update status of multiple Drupal sites. If you're familiar with Nagios and NRPE, it's a little bit like that, but for Drupal modules, and more lightweight. Note the server uses MongoDB, so you need to install that, plus the PHP extension, and it communicates with a public key-pair, though it should set that up itself.
Tested by me
Drupal 8 support