Phobos transits as seen from Mars and associated shadows on the surface.
Because they look so familiar yet so incredibly alien. Technically these events are still annular solar eclipses, so I think the shadow in the Global Surveyor photos should be called antumbral rather than "penumbral" as stated in the photo captions.
The "Lunar Eclipse from Mercury" one is definitely up there too.
Oh, and while we are talkin eclipses,
let's not forget this recent beauty.