Rhea eclipsed by Dione
A rare event in the solar system, captured on Feb. 20 by Cassini: A partial eclipse of two of Saturn's moons, Rhea (rear) and Dione. As with the sun and the moon during eclipses on Earth, the two are of different sizes but their distances cancel out the difference, making them appear the same size. There's a nice story on it here on a blog by MSNBC's Alan Boyle.
You can watch the full eclipse as a Quicktime movie here.