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. Probably the easiest way to start your first DIY Phone Gadgets Project is to use infrared signal. You can purchase a cheap IRDroid module here:
3. IOIO for Android:
4. UDOO: the BEST solution so far to combine Android and Ardunio seemlessly.
5. Arduino + USB Host Shield from Microbridge project:
6. PIC24 board from Microbridge PIC project:
7. Google ADK (Android Open Accessory Development Kit):
8. 3.5mm Stereo Audio Cable from TRRSTAN cellphone robot:
9. Bluetooth+Arduino+Android Library from Amarino:
10. The PhoneDrone Board (Android to PPM RC interface board from DIY
11. Android ADK with a standard Arduino Uno and USB Host Shield:
12. Native USB host support (Android 3.1 or above) to control simple USB peripherals:
13. 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.
14. Android+Raspberry Pi+WiFi dongle. It's really amazing that this tiny pen-sized Linux PC costs only 25 US dollars and it runs Linux! The combination of Raspberry Pi's size, price and performance makes it a nice candidate of our DIY gadget brain, providing diverse interactions with Android.
15. The Electric Sheep board from Sparkfun, a development tool (similar to the Arduino Mega ADK) for creating custom Android accessories.