XBee (Wireless Communication)
http://www.maroon.dti.ne.jp/koten-kairo/works/dsPIC/xbee1.html
http://kousaku-kousaku.blogspot.com/2008/10/arduinoxbee-shieldprocessingxbee.html
Arduino<->Arduino
Arduino<->PC
Firmata (Processing <-> Arduino)
http://kousaku-kousaku.blogspot.com/2008/12/processing-arduinofirmata.html
Server <-> Processing <-> Arduino