There are 3 repositories under raspberry-pi-hat topic.
Raspberry Pi powered High Altitude Weather Balloon
2020: Proto-driver and example applications for Teensy MCUs interfaced to the Wolfson WM8960 Audio DAC with a headphone and stereo power amplifier.
Raspberry Pi hat for CraftBeerPi
Raspberry Pi HAT with a Silicon Labs EZR32 Sub-GHz MCU
Design files, firmware and linux scripts for my /clever/ serial terminal, the Whitterm-220 - WT-220.
An uninterrupted power supply HAT for Raspberry Pi 3b+ and prior b+ models.
Raspberry Pi HAT with MCP3424 ADC, 1-wire and I2C interfaces, and 2 auxiliary switched outputs
Raspberry Pi Zero sensor board
Power Switch for Raspberry Pi
Pi-Lite Arduino library and RaspberryPi Python Example code