SerialControl is a simple open source (GPL2) firmware for the Arduino platform, that allows to remote control Arduinos over a serial connection. The implemented protocol can be used in a XBee peer2peer network. Multiple Arduino modules can be addressed by an ID. The firmware wraps most IO commands of the Arduino language to make the output pins controllable through a serially attached computer.
http://github.com/UrsusExplorans/SerialControl/ (there is a download link in the source/master tabs)
To install SerialControl, just copy the source code directory "serialControl" into your sketchbook folder. Open the Arduino programming environment and load the serialControl package. Adjust the "ownID" variable to your needs. This variable should be unique on the Arduinos, you want to address. Now the firmware can be compiled and transferred to the Arduino. Keep in mind that SerialControl requires the "String library" (http://arduino.cc/en/Tutorial/TextString) to be installed.
After installing the firmware on your Arduinos, you can use it by transmitting commands over a serial connection (e.g. wired or XBee). For first experiments you can use the terminal, built into the Arduino programming environment.
As you might see, the commands have all the same structure: