I've seen this photo before, but NASA's Astronomy Photo Of The Day featured it yesterday:
Image credit: Cassini Imaging Team, SSI, JPL, ESA, NASA
It's a photo taken by the Cassini spacecraft as it orbited Jupiter a few years ago. That's Io in the foreground, a moon that is roughly the size of our own Moon and orbits at about the same distance.
Click on the image credit link to be taken to the APOD page and a more informative caption.
UPDATE: The April 10 APOD is pretty cool, too. It's copyrighted, so I won't reproduce it here, but it's worth checking out. (h/t Suzanne )