This is a handy AVR programming cable that incorporates both the 10-pin and 6-pin ISP programming interfaces.
With the GPS Shield you can add GPS functionality to your Arduino.
Just need to make some motors move with your Arduino? Here's a simple way from SparkFun - the Ardumoto!
The Voice Shield™ (VS) is an analog audio shield for the Arduino that allows you to play audio sound byte.
This is a 7-segment shield for Arduino NG/Diecimila/Duemilanove and many Freeduino boards.
The Joystick Shield sits on top of your Arduino and turns it into a simple controller. Control man, get some CONTROL! (PCB only - buttons/components not included)
This is a simple to use USB AVR programmer. It is low cost, easy to use, works great with AVRDude, and is tested under Windows, Linux, and MacOS X.
The USBtinyISP is an inexpensive kit that emulates the AVRIsp in-system programmer. It's an inexpensive way to start "burning your own code"!