It only flies at 30 mph, but what it lacks in speed it makes up in efficiency. This aircraft from Solar Impulse, designed to demonstrate the potential of renewable energy, stayed aloft for 12 hours 59 minutes without consuming a single drop of avgas. This article by Gizmag notes:
Solar Impulse is an astonishing feat of engineering. It has a wingspan of over 200 feet (61 m) yet it weighs only 1600 kg (3,527 lb) and carries almost 12,000 solar cells which supply all of the energy required to keep it aloft.
The following video shows this amazing aircraft during the final leg of its historic voyage.