During today's opening keynote for Google I/O, they touched on all the major topics that folks were anticipating including Android Ice Cream Sandwich, Google Music, and Google TV, but they also added some unexpected developments. One of them being their new Android Open Accessory initiative, that will allow developers to create their own hardware accessories that can be controlled by Android.
The Android Open Accessory system is built on open-source Arduino. It will will allow externally accessories connect to an Android-powered device via USB and eventually Bluetooth. This could essentially mean the start of a whole slew of "Made for Android" devices such as docks, speakers, or even an exercise bike. This opens up a whole world of possibilities now for external accessories to work with Android.
Google is offering an Android Device Kit or ADK for developers. The kit comes with a sample implementation in the form of a USB accessory along with all the hardware design files, the code for the accessory's firmware, and the Android application that interacts with the accessory. To get involved with the Android ADK, visit here.