There are a good number of companies and enthusiasts who produce open hardware designs.
These notably include Adafruit, Sparkfun, Olimex MLab Zio Plus a variety of individuals on Tindie , OSHWlab or Kitspace
Using electronics typically involves connecting an MCU to various sensors, peripherals or actuators. There are a few standards that try to combine these in an abstract way. Notably Qwiic or Grove for I2C and UEXT which adds SPI. There is also Mikroe Click or Breakout garden but these boards tend not to be open hardware. The problem of incompatible I2C standards is documented here
There are also the ‘stacking’ standards, notably Featherwing, PyEC, M5stack , the RPI 40 pin header or Beagle Bone.
I’ve developed boards in both the M5stack and Feather footprints, but the Feather is small and has limited pins and the M5stack header is hard to source. I explored for a while whether the stacking 40pin header used by the Pi can be useful in a microcontroller context. There have been some recent attempts to do this with the RPI2040-based M10 or ESP32-based EsPiFF (which additionally reuses the Pi form factor). In both these cases you are tied to the MCU offered by the designer with no flexibility to change.
BeagleBit
Microcontroller concept, based on Sparkfun MicroMod system, expanded to allow for 30mm boards Hopefully additionally including the fully open hardware Core-V
(Optional Ethernet, Capable of running Meshtastic with UEXT LoRa)
Featherwing concept
(Would need to be top board to stack, Feather header, 2 Pi hat pinout)
UEXT/ Mikroe/ Grove/ Qwiic concept
(Directional distance sensors for robotics. Adaptor for Qwiic)
Micromod Function cards (Ethernet, GPS, RTK)
Slightly off the Microprocessor topic, A board that takes a Raspberry PI CM4 or compatible processor
Robotics cape exists
Motor/ Servo cape (I2C) exists This would fit on a Zumo or similar robot
Grove exists
Beagle Proto exists
Beagle > RPI header exists (But is closed hardware)
Plus many more Beaglebone Capes
Accessories & Sensors
All the Featherwings
Older abandoned idea – using the Pi header
two of the 40pin Pi header. One would use the standard Pi GPIO With the other optimised for Microcontrollers.
Pi Square carrier 71mm x 71m?
M2 Processor slot
Feather pin header
Qwiic, UEXT, USBC, JTAG
Feather / UEXT daughter board
Feather wings
6x UEXT
Zumo robot daughter board
A board that will fit on the Zumo chassis
With components compatible with the Smartcar Shield software
MicroMod function board
MicroMod function daughter board
Mikroe Click board
2x Mikroe click
Battery Board
3x 18650
USBC charge
5v/3v
CM4 board
A board that takes a Raspberry PI CM4 or compatible processor
Proto board
Prototyping space