Want a tidy AVR-ISP compatible programmer that also acts as a USB to TTL converter? Pololu's Orangutan USB AVR programmer fits the bill!
The Video Game Shield is an Arduino add-on shield to make your own video games, including graphics, text, sound effects, and music!
The LoL Shield is a build-it-yourself kit that creates a charlieplexed LED matrix for the Arduino
The SparkFun VoiceBox Shield adds all the capabilities of the SpeakJet voice and sound synthesizer to your Arduino Project via serial commands.