The Sanguino board is a Arduino-derivate offering a more powerful CPU, more pins and more FLASH. All this, while being compatible with existing Arduino software. It is not physically compatible, so shields will not fit.
The core of the Sanguino board is the ATmega644p chip, a bigger brother to the ATmega328 of the Arduino UNO board. The Sanguino board provides, 32 GPIO-pins, 8 analog pins and 6 PWM pins. On the memory side, 64kB of FLASH and 2kB EEPROM provides non-volatile storage, while 4kB of RAM provides space for your stack and variables.
The Sanguino can be bought or built. For the builders, instructions are provided for a soldered custom PCB, a strip-board style construction and even a breadboard version. For the tinkerers out there, a break out sheild is also available. This way, you get easy access to IOs via terminal blocks for attaching your wires.