The IOIO (pronounced "yo-yo") is a board specially designed to work with your Android 1.5 and later device. The board provides robust connectivity to an Android device via a USB or Bluetooth connection and is fully controllable from within an Android application using a simple and intuitive Java API - no embedded programming or external programmer will ever be needed!
The IOIO board contains a single MCU that acts as a USB host and interprets commands from an Android app. In addition, the IOIO can interact with peripheral devices in the same way as most MCUs. Digital Input/Output, PWM, Analog Input, I2C, SPI, and UART control can all be used with the IOIO. Code to control these interfaces is written in the same way as you write an Android app with the help of a simple to use app-level library. In other words, you can combine the awesome computing power, Internet/Bluetooth connectivity, touch screen, and a variety of sensors from your Android device with the ability to easily add peripheral devices to interact with the outside world. Also, using the IOIO does not require any hardware or software modifications to your Android device, thus preserving the warranty as well as making the functionality available to non-hackers.
The IOIO acts as a USB host and connects to most Android devices that have USB slave (device) capability. So far, the devices proven to work with this board are: G1, Nexus One, Nexus S, Motorola Droid X (among many others now).