Ask HN: Cool Arduino projects

I'm looking to get into Arduino (or We-IO) and in need of inspiration.

What are some cool, small, DIY projects you saw, or wish to see utilizing the Arduino micro controller.

Thanks!

Shameless plug: http://sudharsh.me/blog/2013/12/07/jenkins-build-status-on-a...

Everyone out there seem to start out with a CI traffic light using Arduino. This is no exception :).