The Arduino ADK is a microcontroller board based on the ATmega2560 (datasheet). It has a USB host interface to connect with Android based phones.
It is compatible with Android's Accessory Development Kit examples. It has 54 digital input/output pins (of which 14 can be used as PWM outputs), 16 analog inputs, 4 UARTs (hardware serial ports), a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.
The ADK is based on the Mega 2560.
For information on using the board with the Android OS, see (Google's ADK documentation)