glights is a program that allows you to control lighting equipment from your computer. This is done using the printer port and some external switching and driving circuitry. You'll have to build the circuits yourself, but they are not particularly complicated.

It offers the following features:

• Control up to 8 lamps
• Run various patterns over them (currently 5 different patterns are included)
• Change the speed
• Invert/reverse a pattern either manually or automatically
• Automatically switch pattern
• Pick individual lights to stay on permanently

Building the circuitry

Have a look at http://www.discolitez.com/circuits.shtml, their circuits work pretty well (I'm using the "TRIAC Drivers" model myself). Also, their warnings and disclaimers apply.