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.
This Arduino shield rocks a socket for wireless modules, such as Digi Xbee, so that you can float your bits and bytes of data through air with the greatest of ease.
Need space to add custom circuits to your Arduino MEGA? Here's the open proto shield!
The USBtinyISP is an inexpensive kit that emulates the AVRIsp in-system programmer. It's an inexpensive way to start "burning your own code"!
Sometimes it would be very nice to be able to add connectivity to your Arduino/Freeduino project. The Adafruit Ethernet shield kit lets you add network connectivity!