Hydra is an Emacs library which provides tools to build composable keybinding micro-systems. It allows you to define, basically, easily accessed minor modes such as a "window management" minor-mode, or quickly run interactive commands like toggling variables or doing text transformation, etc. It's a big hairy macro, and it works really nicely. I describe its usage in pages which link to this one.
So for example, when I am presenting Emacs to others, when I am
screensharing at work, or when I am streaming my desktop online, I want
to increase my font size so that it's more legible. With
hydra-zoom, I hit
<SPC>zgggggl to make my font size increase
by a factor of five, and then decrease it once I inevitably realize I've
gone too far.
'evil-leader (with-eval-after-load (defhydra hydra-zoom ()"g" text-scale-increase "in") ("l" text-scale-decrease "out")) ("z" 'hydra-zoom/body)) (evil-leader/set-key