Arduino is famous development board for prototyping your microcontroller project. But, it will expensive if you need lots of controller boards for a larger project or you have multiple concurrent projects. There are some great, cheaper alternatives (Boarduino, Freeduino) but the costs still add up when you need many of them. For this problem, you can try uDuino, a very low cost Arduino compatible development board by tymm. You can build it for $10.
Low Cost uDuino
uDuino documentation and schematic(PDF)