A very well supported mod that adds a lot of stuff to work with to create logic systems. You get buttons, witches logic gates and even an FPGA you can freely program to do a lot of cool things. You can also have a ctroller that can run (mostly) arbitrary Lua code when triggered in a defined way and that can output signals that can be processed by other components.
It also interacts very well with Digilines and Technic and allows complex machinery with text output on displays.
This is one of the few mod’s I always enable when starting a new world or trying things in singleplayer.