* A Lego compatible case, that can accept an Arduino board.
* A range of Lego compatible sensors and actuators
* Other components following the theme of integrating open-source with Lego.
* and so on...
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Edit: formatting
I started the Bitbeam project because I was prototyping some things in LEGO, but when it came time to figure out how to make many of them, I wasn't too excited about the cost of LEGO... but I was happy with my LEGO design. So I decided to port LEGO dimensions to materials that are easier and cheapier to source myself.
Some that are blessed by Lego: http://www.hitechnic.com/products
... and some that are not: http://www.mindsensors.com/ http://www.techno-stuff.com/
Also, this exists, targeted at the education market: http://www.vernier.com/products/interfaces/bta-nxt/