When I first browsed the Arduino site I am sure I saw a repository for designs. I can find the software library share but not the hardware share.
I have used an Atmega 328 and Arduino to build a chicken coop pop-hole opener. The design is based on the Arduino with motor shield but includes a current measurement facility which might be useful to others. It uses a Light Dependant Resistor to measure light levels and works out what the season is and what the time of day is to open and close 4 pop-holes at appropriate times.
The design is released under GPLv3 and it, along with the research notes are posted at www.electrichen.co.uk
Where on this site should I post the link please?