Microsoft’s WorldWide Telescope (“Spring Beta”) has just launched and is available for download for Windows XP and Vista. I was able to take it for a quick spin on a very underpowered system before it crashed but even so it left a truly stunning impression. This is an amazing tool. If you have a well-specced Windows machine (not Parallels or VMWare on the Mac like me) then drop everything and lose yourself in this for the next few hours.
Compared to Google Sky, you can really see the advantage of a building a dedicated astronomical browsing tool. I have to agree with Scoble (much as I hate to :-) — this is easily the most impressive thing I’ve seen Microsoft do in a long time, and it stands head and shoulders above the competition. It may even force me to go back and reinstall Windows on my Mac via Boot Camp, because I suspect that’s the only way I am going to get to use this application. (Thanks Jonathan for the heads up)