The Lots of LEDs Shield gives you total control over 126 SMD LEDS, creating a programmable display for your Arduino.
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!
The SparkFun VoiceBox Shield adds all the capabilities of the SpeakJet voice and sound synthesizer to your Arduino Project via serial commands.
Here's the official Arduino Ethernet Shield With PoE Module
This Arduino shield interfaces the Pololu Wixel wireless modules to your Arduino-compatible project, enabling simple wireless programming, debugging, and control using standard Arduino software.
Here's a Pocket Marquee Bundle for setting up your own portable sign or displays. Design your own animations/messages and show them off wherever you go.