Re:NASA WorldWind also ready for Linux and MacOSX (Score 2, Informative)

There is also a World WInd clone written by Pat Murris - WW2D Plus One (homepage) written in Java.

It's obviously quite simplistic however it is a 3D world viewer and can use the same imagery as World Wind, so it's a start and certainly something to use if you want to see the good-looking Next Generation Blue Marble imagery for one thing :)