Arduino is a very nice prototyping platform but, Although not very expensive, costs at least € 20 (25 USD). The CSEduino is the answer to a very low cost DIY Arduino like board.
The CSE stands for Cheap, Small and Easy to build.
With a very small budget of up to € 3.5 (5 USD) in parts it is possible to build this board with the required components. To be small some optimizations were made to the stripboard to accomplish its reduced size.
João is also doing a talk on the same topic is should look into.