Strobist. I went a little over his $10 boast, but I needed a couple of items not on his list.
I've been saving up materials for a while, trying to figure out when I'd have the time to do it, and a couple of hours finally presented themselves. The box is an old monitor box I've been saving for years to move our computer's monitor (which will probably be replaced soon with a flatscreen). The panels are standard tissue-paper, but I taped over the entire surface with clear packing tape to make it more sturdy since there are plenty of little people around the house that would easily put a hand or foot through the side in curious exploration.
As far as additional expenses go, I used about $3 in posterboard (needed two white sheets, and I have one black sheet cut to a 20x20" square that you can't see here). The biggest outlay in money was the $20 daylight-balanced bulb which I stuck in a $5 clip-on-lamp from the hardware store. I just don't have any other good source of light in the house (especially this time of year when it's grey and grim and natural light is hard to come by). I don't even have a removable flash, so some sort of light was necessary.
I've already started to use it and it's so much fun! I'm able to get some glamour shots of my yarn, and soon I'll be putting my finished projects in here to show them off. It was completely worth the effort!