This tutorial shows you how to connect Raspberry Pi and Arduino using I2C communication, how to configure it. Combining them does not require your USB port.
Raspberry Pi to Arduino shields connection bridge
Introducing the PiDrive, a high capacity Solid State Drive (SSD) expansion card for the Raspberry Pi B+, A+, and B+ v2!
Pi Tank 3D Printed Open Source Smartphone Controlled Raspberry Pi Robot
DiddyBorg - The most powerful Raspberry Pi robot available | PiBorg
Cpcdos, Raspberry Pi, Arduino Plus
MeArm robot arm using the Arduino or Raspberry Pi as controller https://hackaday.io/project/181-mearm-your-robot
RaspberryPi Arduino Webcam laser range finder