TinyPICO
TinyPICO is the world's smallest, fully-featured ESP32 development board in a package smaller than your thumb!
TOMU
Tomu is a programmable computer that fits inside a USB port. It is user-programmable, has two buttons and two LEDs.
Teensy 4.1
Teensy 4.1 features the fastest micocontroller and an expanded set of powerful peripherals in a 2.4 by 0.7 inch form factor.

Teensy 4.1 features the fastest micocontroller and an expanded set of powerful peripherals in a 2.4 by 0.7 inch form factor.

Teensy 3.2 is a powerful device in a small form factor. It features a 72 MHz Cortex-M4 processor and 5V tolerant pins.

ANAVI Thermometer is an open source hardware, Wi-Fi development board for measuring temperature that's powered by an ESP8266 processor.