Every user of Arduino and similar devices who do not know PJON should take a look. Especially the SoftwareBitBang strategy is genious, allowing lots of devices to communicate over a single wire (plus ground) over long distances on a bus, without wasting money and space on communication shields.

I use PJON as a basis for a library I created for my home automation, and it works flawlessly (https://github.com/fredilarsen/ModuleInterface).