These tools and solutions are among the most popular ways to interface an Android device with the physical/eletronics/mechanic world. But sure, you can help us to include more if you see something else:)
1. Bluetooth module+Arduino+Android Application from Cellbots :
2. IOIO for Android:
3. Arduino + USB Host Shield from Microbridge project:
4. PIC24 board from Microbridge PIC project:
5. Google ADK (Android Open Accessory Development Kit):
6. Chicken Haha Android Serial Connector:
7. 3.5mm Stereo Audio Cable from TRRSTAN cellphone robot:
8. Bluetooth+Arduino+Android Library from Amarino:
9. The PhoneDrone Board (Android to PPM RC interface board from DIY
10. Android ADK with a standard Arduino Uno and USB Host Shield:
11. Native USB host support (Android 3.1 or above) to control simple USB peripherals:
12. IOIO+WiFi dongle. It's a project at its very beginning. IOIO's creator Ytai has just finished adding Bluetooth to IOIO, and expecting experienced people helping him writing WiFi dongle drivers. Please contact him if you have a clue.
13. Android+Raspberry Pi+WiFi dongle. Well this is just our dream, as it will be available in late November 2011. It's really amazing that this tiny pen-sized Linux PC costs only 25 US dollars and it runs Ubuntu! The combination of Raspberry Pi's size, price and performance makes it the best candidate of our DIY gadget brain, providing diverse interactions with Android. We will definitely have a try:
14. The Electric Sheep board from Sparkfun, a development tool (similar to the Arduino Mega ADK) for creating custom Android accessories.