Oomlout have come up with a neat design for a (nearly) free project box for Arduino. It's printed on 210 gsm card stock; you print it, cut it, fold it, and voilà - a simple but functional home for your Arduino creation.
I'm working on a project right now that could use one of these. I'll pop out for some card stock and have a go.
Update: My local Stationers (WH Smith) have packs of white 220 gsm card stock for a little over £3 , and they also have mixed packs of coloured card. So you can make yourself a coloured project box if you want.