I worked at a company that had a powerful legacy software that was meant to be configured once and run full screen. At some point it needed a custom interface

I worked at a company that had a powerful legacy software that was meant to be configured once and run full screen. At some point it needed a custom interface and the ability to reconfigure itself on the fly. Adding in a GUI was not a reasonable option. We ended up writing a GUI (in gtk) then using Xembed to embed the other process and communicate via a Unix socket. What would have been a major rewrite (and likely a port to a different language) ended up being a few a days project and worked beautifully. It really showed me how powerful X11 really was.