Some diagrams illustrating how to use the tip120 transistor to control 12 v (actually up to 60v dc)
12 v lamp:
DC motor (only difference is the diode connected between collector and emitter.
A detailed diagram for the tip 120:
A simple example code for on/off using max/msp and arduino from Tom Igoe´s pages works fine to test the transistor as a switch.
Another example combining max/msp and arduino to fade a lamp for instance.