Computer Christmas has many articles showing you how to do this. There's a how-to for a 320 channel controller. All I need is 8 channel on/off (no dimming), so a simple parallel port relay box is perfect for me. This how-to tells you how to building one. But actually you don't need to build a relay box yourself, there are parallel port relay kits sold online for cheap. You just buy the kit and connect to electric box.
So Here's how to build a Python powered Xmas light controllers for $40(more or less).
1, get a parallel port relay kit for about $32, for example: here or here. You can get it assembled or in module form for a few more bucks. I didn't want to spend much time assembling so I got a module.
2, get electric box and outlets from Homedepot or Loews for total $8. Again I'm lazy so I just got a power strip from Walmart for $4, but it only has 7 outlets, I couldn't find one with 8.
3, Wire the kit to your electric box.
4, Connect to computer and fire up Python (see my previous post).
Video of testing session.