Y'know, when I saw the McNaught animation from STEREO I was totally enthralled, and I figured it'll be a while before they can top that. Bzzzzt. It took them a day.
STEREO-B caught what astronomers call a transit -- the passing of one celestial body in front of another -- but in this case it's OK to call it what it is: a solar eclipse, when the Moon passes in front of the Sun. On Earth, the Sun and Moon are about the same size in the sky. Every person on Earth, since as long as there have been people, have seen the Sun and Moon about the same size in the sky. The only exception I can think of were the Apollo astronauts, who, as they approached the Moon, saw it as being bigger than the Sun. But no one, ever, has seen the Moon pass in front of the Sun when the Moon was significantly smaller than the Sun. No one. Until now. WATCH THIS VIDEO
It's, well, it's impossible to do it justice. This is quite simply one of the most beautiful things I have ever seen. It's not just the video itself, it's knowing what I'm seeing. The seething surface of the Sun, the sunspots (glowing white in this ultraviolet view), the fantastic prominence on the lower right side that you actually get a 3D feel for as the Sun slowly turns, the sparkling granules caused by convection cells at the surface. Then the Moon moving across the disk... Wow. Watch it in high-res. It's worth the wait. [Note: I updated this post on October 17 2010, to include the link and embedded YouTube video.]
Update: Oops! I meant to mention that Saturday night there is a lunar eclipse (where the Earth's shadow falls on the Moon) which favors the eastern part of the U.S. and Europe. Sky and Telescope has the info you need to see it. Interestingly, the Moon will rise in eclipse for me in California. I've never seen that before, so it'll be cool-- and it'll even be clear tomorrow night!