The amazing thing, to me, is that the Moon's diameter as viewed from the Earth is almost exactly the same as that of the Sun. I've heard that, of the moons in the Solar System, only a handful subtend the same arc as the Sun when viewed from their primary's surface (though of course "surface" is a tricky concept when we're talking about the gas giants), and of those, I don't think many of them are spherical. The kind of diamond rings we get during eclipses are probably quite rare.
jiawen writes "Data from the Spitzer Space Telescope has been used by researchers to make the first-ever map of an extrasolar planet. It's a weather map, more precisely, showing temperature variations over the surface of a Hot Jupiter, HD 189733 b (more, more). It really is hot: even the coldest regions are about 1200 degrees F."