Microsoft has just released a public beta of the WorldWide Telescope.
"The WorldWide Telescope (WWT) is a Web 2.0 visualization software environment that enables your computer to function as a virtual telescope—bringing together imagery from the best ground and space-based telescopes in the world for a seamless exploration of the universe."
You can download it from here for PC and Mac (with Boot Camp and Windows). Requires .NET 2.0 and DirectX.
First try
I tried the WWT very briefly and initial reaction was a bit "meh", but the more I played it, the more I liked the experience. I didn’t have time to try all the features, but I like the intuitive navigation and "flying effect", when choosing destinations. It’s easy to find familiar destinations through collections and cool just to explore randomly.
It is a really nice experience for those interested in astronomy and a very good learning tool. There are many community options to share tours etc.
Here are some screens:
Yes, this is the product that made Scoble cry 
