Microsoft WorldWide Telescope has been around since February 2008 as a standalone app, sort of like Google Earth. It allows you to pan around space thanks to images taken from Earthbound telescopes as well as the Hubble, but that's not the only feature. It also gives a very Google Earth-like option to zoom around our own planet, it has 3-D rendered models of all the other planets in our solar system, and even gives access to Mars Rover panoramic shots.
Of course, Google Sky, which despite minor differences in processing and controls is essentially the same thing, came out about eight months before WorldWide Telescope. Just look at it above, and compare it to the new WorldWide Telescope, pictured up top. Similar, aren't they? But in the years since Google Sky's release, as standalone apps have fallen by the wayside in favor of in-browser apps, Sky has come to the browser, while WorldWide Telescope has languised as a Windows-only app. Not anymore.
Now, Microsoft has decided to release a WorldWide Telescope Web client within its pre-existing Bing Maps app. It works quite well, though it is jerkier and more processor-intensive than Sky, as well as, frankly, not as pretty. It's also lacking a mobile version, like Google's Sky Maps for Android, though of course Microsoft is in a peculiar in-between time right now regarding mobile OSes, having already declared Windows Mobile 6 dead but having not yet released its first Windows Phone Series 7 handset.
There's nothing wrong with WorldWide Telescope--it's actually an amazing and wondrous source of information, and if you'd never seen Google Sky you'd be blown away. But we have seen Google Sky, so we're left to ask: What's new, Microsoft?