We've built our keyboard firmware on top of Arduino, the world's most widely used open platform for embedded hardware development. The global Arduino community is rich, vibrant, friendly and incredibly prolific.
Specifically, the Model 01 is built around the Atmel ATmega32U4, a powerful AVR microcontroller that's particularly good at being a USB device (like a keyboard or a mouse)
Our Keyboard firmware is freely available on Github. We've designed it to be simple and reasonably straightforward for someone new to Arduino and embedded programming to get started with.
We've been typing day in and day out on keyboards running our firmware since late 2013. The code didn't change much in the first half of 2014—we were focusing on hardware design and learning about manufacturing.
While we primarily designed our firmware to be useful to folks with our keyboards, we know of a few hobbyists who are already using it for their own creations, too.