SADbot gets its name from the source of its power: the sun. Since the motors are powered from a battery charged through a solar panel, they will run only if it has been sunny enough to store solar energy in the battery. When the battery runs low and the SADbot motors stop running, SADbot appears sad because it has to wait for the sun to come out before it can keep drawing.
Everything you need to know is in this Instructable, but if you've never used an Arduino or worked with photocells and motors, you might want some background material. This the last project in my book, Making Things Move: DIY Mechanisms for Inventors, Hobbyists, and Artists, so all the background material can be found in there.
SADbot has already been listed as one of The 10 Robots That Rocked in 2010 and one of The 10 Coolest Kickstarter Projects of 2010. SADbot also got some love from Adafruit Industries, IEEE Spectrum, True/Slant, Robot Living, and gizmowatch. And when Ben and I showed it at Maker Faire NY in September 2010, our booth won a blue Editor's Choice ribbon! Hopefully you'll make one and comment about it - the world needs more robot art.
Also see the Flickr set for higher-res images.