Emerald is a personal project of mine to assist me in maintaining my small (and private) Minecraft server. I also wanted to learn a bit more about making web-based applications, so I decided to implement it with Ruby and Sinatra (a web library for small scale apps). However, currently the software is not in use, due to the increasingly complex requirements for my purposes. Effectively, my needs grew out from a simple management panel.
It is available on GitLab for public use, although there may very well be alternatives that are both simpler to install and more advanced in terms of features. Still, it has proven its ability in lightweight daily use, with some field-specific extensions (e.g. kerberization, when I ran a FreeIPA master)
See the GitLab repository, you can get the latest public version from there.