A few years back, I built a wooden PC case that was just large enough for a microATX motherboard with a PicoPSU. It only stayed in the case for a few months before I ended up just screwing the wooden backplate to the wall, and having an open-air machine.
The wooden backplate was recently upgraded to steel to allow expansion cards to seat more happily in the system. I harvested an ATX backplate from a very old PC case. It's easy to drill the rivets out and take the parts you need. A bandsaw is the ideal tool for cutting it down to size, but I used a sawzall and tinsips here.
I recently retired my energy-hungry gaming rig, and upgraded the Woodbox from a Sempron to a dual core A64 X2 @ 2.1GHz. I have a 32MB OCZ SSD, which is plenty of space when carefully managed. There's also a 0.5TB Samsung Spinpoint that stays unplugged unless I need to store/retrieve something. 3GB RAM, PicoPSU, and a Scythe Ninja that I've had for a long time now.
Here's a picture of the computer posing with a GeForce 6800 GT, which is where I'm storing my interesting GPU heatsink.
I have a Sapphire Radeon HD 5670, onto which I could swap that interesting GPU heatsink. But the Radeon adds 10W to my system's idle, so as long as I can play StarCraft on my onboard graphics, I don't need the graphics card.
The system idles at 22W, exceeding my expectations.
There are no fans, and no moving parts when the hard drive's not plugged in. I undervolted to the threshold of instability, then bumped up the voltage two notches.