I've been working on creating a smaller version of the Netduino for a while. I posted a version on the Fritzing website a few months ago, and since then I've made a new iteration. Here is a screenshot of the new version.
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The board is 30mm x 54mm (1.18" x 2.13") and has pretty much the same functionality as the Netduino 2.
It can be powered via USB or the VIN pin.
All the same GPIO pins are there, but I've also added one more 3.3V and one more 5V output.
Compared to the original Netduino 2 it doesn't have IOREF, /RESET, the power barrel or the JTAG. The circuit on the original Netduino 2 which switches between the powerinput from USB or VIN has been replaced by a LTC4411 diode.
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The board accepts the Netduino 2 firmware. Hold down the Firmware button while booting and run Netduino Update tool.
Thanks to Chris Walker for opensourcing the Netduino hardware! I'm including the Eagle 7.1 files if anyone wants to modify, or create their own versions.