Nauthilus is a general purpose authentication service written in Go. It is designed to work as a central place where all kinds of authentication requests are handled. No matter if you need to authenticate users from a mail server or a web page. Services are connected to Nauthilus.
This is an example of Nauthilus, how it is integrated in Dovecot and Roundcube.
The documentation is found on https://nauthilus.org.
The documentation itself is another project at https://github.com/croessner/nauthilus-website. Feel free to make it better! :-)