Just saw a TED talk about these Little Bits, electronic building blocks. Magnet connectors make it easy and fun to safely snap together components. The project is open source so the schematics are available online if you want to make them yourself. I think that it would still be hard to make your own building blocks, what with circuit boards and special plastic pieces, but it seems to me that other people can step up to add more interoperable modules. Not sure the price, but it looks like a blast to experiment with. What do you think?