Arduino boards are great for prototyping. However they get rather expensive when you have multiple concurrent projects or need lots of controller boards for a larger project. There are some great, cheaper alternatives (Boarduino, Freeduino) but the costs still add up when you need many of them.
This is a way, after about $25-$30 initial investment, to build sub-$10 Arduino-compatible boards with very little extra time investment on each.
uDuino: Very Low Cost Arduino Compatible Development Board - [Link]