For those who missed it, NASA World Wind is an open source 3D interactive world viewer, every bit as impressive as Google Earth (GE), ArcGIS Explorer (AGX) and Microsoft Virtual Earth (VE), yet not as popular.

NASA just reseased World Wind 1.4 with a movie-style trailer, opening thus:

"The First Generation of Virtual Globes lets you see your house
Now Get Ready for The Next Generation"

Daniel Baker, a member of the NASA World Wind Open Source community, writes in his blog:

"World Wind is aimed at education and science, all the default imagery is copyright free, you don't need to purchase a license to redistribute the imagery and there are no pro or expert versions to buy, you can happily use World Wind at work or school without fear of any lawyers confiscating your research."