This Citrus-Powered Digital Clock from Florian Dussopt & Julie Girard is powered by electrolysis and the citrus juices of a lemon or orange. My science teacher had one of these and i thought it was cool. There are two spikes (one copper and one zinc) which are stabbed into the fruit and this produces a current using the juice as a transport medium. One slice of fruit can power the clock for a week!

The Design Blog