Programming

My own. My Emacs config grew over years to several thousand lines, and it got to a point where I decided I could write an editor in fewer lines that it took to configure Emacs how I liked it. It's ... not for everyone. I'm happy with it, because it does exactly only the things I want it to, and nothing else, but it does also mean getting used to quirks you can't be bothered to fix, and not getting to blame someone else when you run into a bug.

That said, writing your own editor is easier than people think, as long as you leverage libraries for whichever things you don't have a pressing need to customize (e.g. mine is written in Ruby, and I use Rouge for syntax highlighting, and I believe Rouge is more lines of code than the editor itself thanks to all the lexers)