Emacs
Despite popular claims, emacs is not an editor, but rather an interactive, self-documenting hypertext environment attached to a lisp interpreter.
I use it for programming, reading and writing emails, keeping organized, file management, keeping up with RSS feeds, and much more.
It kinda sucks in a lot of ways, but so far nothing better exists that could replace it.
I maintain two emacs packages.
wayproto.el is a wayland protocol viewer