The ATMegaZero is a microcontroller board based on the Atmel ATMega32U4 microchip, the same chip used in the Arduino Leonardo and Arduino Micro. It was modeled after the Raspberry PI Zero to take advantage of its sleek design and form factor but in the form of a microcontroller board. Similar to the Raspberry Pi Zero the ATMegaZero comes with 40 GPIO pins that can be used as input or output for interfacing devices and can be programmed using the Arduino IDE software. The ATMegaZero comes with a built-in USB which makes the ATMegaZero recognizable as a mouse or keyboard.
Two extant variants at this time: One based upon the ATMega32U4 (older) and one based upon the ESP32-S2 (newer) (CircuitPython compatible).
A cheatsheet for the most popular microcontroller chips' pinouts, connection headers, and cable pinouts.
The SwinSID is a pin-compatible and software-compatible replacement for the Commodore SID chip based upon an Atmega microcontroller. You can drop this chip right into a socket on the motherboard of a damaged Commodore computer, or you can probably drop it into a hardware synth that uses SID chips and hit the ground running. The firmware image and schematics are available for download for free on the website, so you can roll as many of your own as you want.
Plans for constructing an Arduino shield which lets you stack another Atmega 126 or 328 in master/slave configuration. Double the I/O pins, double the interrupts.