mjolnir - Lightweight automation and productivity app for OS X
Mjolnir is an OS X app that lets you automate common tasks using the language Lua. At its core, it doesn't actually do anything besides load up a Lua environment; the real power lies in all the useful modules that you can install.
You write a "config", which just means ~/.mjolnir/init.lua. This file, along with whatever modules it requires, have full access to the built-in mjolnir module, and all Lua modules that you have installed (e.g. from LuaRocks or any way you want to install them).
Comments and Reviews
Mjolnir is great if you're the "roll your own" type and don't mind running 4-5 year old software. Its Github project shows some recent activity, but was last actively maintained in 2014.
Hammerspoon is a modern fork of Mjolnir with a simple, but usable GUI, actively maintained on Github.