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.
This module comes straight from those fine folks at Smart Projects (the creators of Arduino), and sports a 3.3v regulated Digi Xbee slot for easy wireless connectivity (that works with a wide range of other wireless modules, like Bluetooth, RFID, or Wifi Modules), a uSD slot (micro Secure Digital, like this) for onboard data storage, some active voltage translators and a compact prototyping board area for experimenting and adding your own tweaks.
The microSD socket can be pretty useful for working with an SD library to save and load data. If it's used, though, the CS pin is used by the library and won't be available.
As is standard on any good electronic device, it also has blinky things (transmission indicator LEDs), a button (for reset), and a switch (for selecting how to communicate with module, via USB or the board). And because more always equals better, there are also pass-through headers so that you can stack up and use additional shields.
Check out the official page (at arduino.cc) for many, many more notes and usage documentation