Summary: Sanguino is a port of the Arduino core to work with the atmega644p and atmega1284p chips. It is meant to work with the corresponding version of Arduino which is it ported for.
Install:
First download the corresponding version of Arduino, this should match the Sanguino version, i.e.: Sanguino0023r4 would use Arduino 0023, and Sanguino0101r1 would use Arduino 1.0.1.
Unzip ( or pull from repo ) Sanguino
Copy the 'sanguino' directory into Arduino's 'hardware' directory.
i.e.: arduino-1.0.1/hardware/
Arduino should now show the atmega644p and atmega1284p boards.
Source: The default project repository will track all version 1.0 and greater, if you are looking for pre 1.0 code it is in the legacy repository.
More Info: Arduino: http://www.arduino.cc/ Google Code Project: http://code.google.com/p/sanguino