AFAIK it's closed source, it's a perpetual license for $10. You can run it on 5 servers and it offers unlimited updates. I was wary initially but it's a solid, very well made and reliable product.
Surprising how much functionality and configurability it offers via a Web UI and it's all written by one person. They did a Q&A last year to discuss the product:
They also used to be called McMyAdmin for anyone who remembers them from the older days. Was a pretty popular admin panel for Minecraft servers and is where they started. Then they eventually expanded into other server types as well and basically put McMyAdmin under the AMP umbrella. AMP is definitely a good option for a Minecraft server, but is also a good option for many types of servers out there. It's still actively developed and they add support for new types of servers all the time!
Personally I've always been happy with the McMyAdmin license I had, and ended up also getting an AMP license later.
