Alex's Almost Useless Machine is a solar-powered desktop amusement that slowly saws through dowels whenever the light is bright enough to power its motor. It's mesmerizing.
This little machine uses a so called solar engine to drive the motor. This solar engine is able to collect tiny amounts of energy over time and stores it in large capacitors. When the voltage reaches a certain level, it opens up and uses all the stored energy at once to drive a motor. These engines were used to drive tiny BEAM-bots and were quite popular a while ago. A lot of information can be found at beam-wiki.org on how to build them and how they work.