I have always been fascinated with Christmas lights and thought it
would be cool to hack some lights to be powered via my laptop's USB
port.  (It will obviously work on a Desktop PC or Mac as well!)  

So this is what I did-

1.  Get a USB connector cable (preferably from cablesforless.com so you dont get ripped off http://www.cablesforless.com/index.asp?PageAction=VIEWPROD&ProdID=1146).

2.  Get some batterry operated Christmas lights.  These can be purchased at Ace Hardware, Walmart or even your local dollar store.
Get the ones that use a 5 volt powersupply.  (2 batterry or 3 battery ones work nicely.)

3.  Cut the batterry pack off of the Christmas lights.

4.  Strip the USB connector cable and the red and black wires inside.

5.  Solder the red (+) and black (-) USB wires to the positive and negative wires of the Christmas lights respectively.  You may want to verify with a multimeter before you solder to insure you have the correct wires. 

7.  You now have a really cool set of USB christmas lights!  Geektacular!

If someone wanted to take the time to write a driver, you could probably insert a transistor connected to the data out cable (green i think) of the USB cable and write a small application to turn your lights on and off!