Adafruit Motor Shield V1 communication. The old one based on two L293D and 74HC595 chip. Cloned by many vendors.

Hardware sleep to save battery on AVR-based boards.

Nodes to read OneWire DS18B20 digital thermometers

Read values from a quadrature rotary encoder (EC11, optical wheel encoders, and others)

Make your Arduino pretend to be a keyboard

Kinda useful functional nodes

Random numbers generation

Drives stepper motors connected through a step/dir driver board

Text manipulation