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This was a very old project i did in 2005 (when I was 16) before I even learnt programming. The aim was to have lights-devices of 240V controlled using a parallel port of a PC. A USB was not used, since back in the day it was a mysterious thing for me. If I were to redo this project I would use a uC connected with USB or through an ethernet chip bridge but still use the circuit below that converts a 5V signal to a 240V switch. 

The TO-220 packages are the TRIACs, white DIPs are the isolators. Touching one of those will certainly electrocute you!

Electronics:

//more info and credit :

http://www.discolitez.com/index.shtml

The circuit below is using TRIACs to go from a logic signal to a 240V switch. TRIAC is essentially a dual thyristor or a triode and it can only be used with alternating currents of any voltage.  http://en.wikipedia.org/wiki/TRIAC

The circuit consisted of: PC signal -> opto-isolators -> TRIACs -> 240V bulbs

Why TRIAC's?

Why TRIAC? you ask. Semiconductor Relays are expensive, whereas a simple TRIAC and an optoisolated TRIAC driver, such as the MOC3041 from Motorola can still do the job quite nicely.

Opto isolator?

An optoisolator is a device which is used to keep low-voltage microprocessor circuits (such as the very delicate parallel port) and high-voltage (mains) apart. They generally consist of 1 or more Infrared LED's pointing at a detector. The detector is used to trigger the TRIAC or SCRs. Sometimes, as is the case with the MOC3041, there is a Zero-crossing detector also incorporated into the same package.

Which triac should I use?

Generally, to be on the safe side, choose a TRIAC which has more or less double your mains voltage rating. I.e. for 220V use a 400V TRIAC. Check out the pricing on TRIACs to see which choice is economical for you.

Software:

The software was a plugin for winamp that allowed the user to select the frequency responce for each bulb.

more info and credit :

http://www.discolitez.com/index.shtml