Worldwide Telescope is the coolest application I've seen in a very, very long time. I urge everyone to go check this out, I don't think you'll ever look to the sky in the same way again. While it didn't quite make me cry like Scoble, I guarantee it will evoke a sense of amazement as you explore around the universe. For a quick dive into some fantastic imagery, be sure to go through the Collections under the Explore menu, the Hubble set is my favorite so far. Right click at any point and learn about what you are looking at. The best overall feature and the one that I think will change the way astronomy is taught is the Guided Tours feature. Simply awesome!

From a competitive point of view, I can't resist from also urging you to bring up WorldWide Telescope, and the Sky product from those "search guys" side by side. The difference is night and day; they are light years apart (okay enough astronomical metaphors)...

On a serious note, the two applications running side by side sum up the entire debate around SaaS, Software + Services, etc. In fact I won't even comment on it really, go use both apps side by side and see which you prefer. This is great example of the power of software (the WWT client) plus Services (the imagery and data).