I tend to leave my computer on all the time. It's a combination of laziness and need. I sit on line quite a bit, and the kids use it as well. I don't think I'm unique here.
Recently moved to a smaller apartment, and finally noticed the somewhat shocking electric bill. When I was in a house, I used to put it down to the baseboard electric heaters, or the watter heater or something. Now, it appears that it pretty much has to be the computer. With the monitor, printer, speakers and other crap I have going all the time, it has to be sucking up the power while it's just idling along. Putting the computer into screen saver still leaves all the external stuff turned on.
Last Christmas I grabbed a bunch of solid state relays off Ebay to use in a computer controlled lighting display. It's came out pretty cool, and left me with 8 spares.
For those who don't know, a solid state relay is something like an electronically controlled switch. More info here: http://relays.globalspec.com/LearnMore/Electrical_Electronic_Components/Relays_Timers/Solid_State_Relays
When I was looking for ways to control my Christmas lights, I came across some software for controlling the LPT (printer) port pins. The raw software is available here: http://neil.fraser.name/
After thinking about my power problem for a while, I decided to try and rig something up to turn on and off a bank of power sockets triggered by the screen saver.