Here’s a blatant plug for the yet-unavailable program from Microsoft Research: World-wide telescope (WWT). There’s a presentation about it from this year’s TED conference, too. Availability of the free program is in “Spring 2008″.
Google Earth already has something similar, and has been available for a while: Google Sky (select “View” then “Switch to Sky”; or click the little Saturn icon at the top of its window). Google also lets you explore some other celestial bodies in detail: Mars and our Moon.
Perhaps Microsoft’s marketing machine is just that much bigger? The local planetarium at Intech (opening at Easter!) has been playing around with WWT in alpha form for a little while now.