Explore the universe with your computer
WorldWide Telescope could be regarded as Microsoft's direct competitor to Google Sky, but I wouldn't agree with that.
WWT does contain space imagery that enables you to observe starts, planets, constellations and other celestial elements without having to buy an expensive telescope, that's true. In that sense, it's similar to Google's space browsing tool. But in my opinion, this app is much more intended for professional astronomers – or at least serious astronomy fans – rather than for people who just want to take a sneak peek at our planet as if they were taking a ride on a NASA shuttle.
Don't get me wrong here: there's nothing wrong with being a serious astronomy app. I'm just warning you in case you expect something else. Other than that, WorldWide Telescope is a great tool that all astronomy fans are going to love. It includes loads of information and also lets you download guided tours from the program itself to learn more about a certain topic.
You can freely move around the sky with your mouse, but I found this a bit confusing and soon preferred to use the image galleries under the Explore menu. These galleries were a bit disappointing though, because you can perfectly see the boundaries of the image you're viewing and so it doesn't feel like a real telescope anymore.
Worldwide Telescope is not a space browser like Google Sky; rather than that, it's a comprehensive astronomy atlas with loads of data, photo galleries and freely downloadable guided tours.