Have all of the digital logic and analog circuitry necessary to implement a full-speed USB peripheral/host controller with your Arduino. Use it to interface and control any USB slave device - thumbdrives, digital cameras, bluetooth dongles, game controllers, Android phones and more!
Here's the official Arduino Ethernet Shield With PoE Module
The Adafruit LCD backpacks reduce the number of pins needed to connect to an LCD. LCDs are a fun and easy way to have your microcontroller project talk back to you.
Because the Arduino (and Basic Stamp) are 5V devices, and most modern sensors, displays, flash cards and modes are 3.3V-only, many makers find that they need to perform level shifting/conversion to protect the 3.3V device from 5V.