Fun, neat thing Spark did. They built an open source Nest thermostat in a day:
All in, we spent about $70 on components to put this together (including $39 for the Spark Core); the wood and acrylic were free. We started working at 10am and finished at 3am, with 3.5 engineers involved (one went to bed early), and the only work we did in advance was order the electronic components.
Great visual post too.
/via Mike Rundle on Twitter.