We all know that Google announced in May 2011 a brand new Android Open Accessory standard. With the arduino-friendly ADK board and some Android APIs you can add any accessory or external hardware to your Android phone or tablets (Android 2.3.4 or above).
Please visit Google's official site for more details on how to add accessories to your Android devices: http://developer.android.com/guide/topics/usb/adk.html
That's a lot of text! Before diving into the boring code, many people would like to have a quick demo or test with their own phones. You just need to buy an ADK board and download this standard ADK DemoKit Application from Android market.The application is 100% compatible with the standard Google ADK DemoKit and is recompiled by the DIY Phone Gadgets community for easy tests:
Download ADK DemoKit From Android Market