Emacs is like a hackers' playground. It's an editor, it has a Lisp, networking support, tools, IRC, Email, Usenet and more by default, a PDF reader, a calculator (with gnuplot support it has graphs), a doc generating platform, an agenda, a silly chatbot...
One of the oldest open source projects in the existence?
It makes building custom UI workflows so easy, I think it’s both obvious and flies under the radar.
With Emacs widgets and some settings at init.el (for speed) you can almost create a grude GUI for something with all the power of Elisp (and disabling nearly all keybdings OFC).