Recently, I downloaded a 3D map viewer, called "Dapple", built on top of NASA World Wind .NET version. Geosoft, Inc., took the .NET World Wind and extended it, as well as modified it in general, so as to appeal more to scientific users.
I love that Dapple has a clean user interface combined with an easy ability to add WMS layers. Also, one can use a "Dap Server", which Geosoft offer as an alternative means of storing and sharing geographic data.
Dapple is licensed under the very permissive and developer-friendly software licence known as MIT License. This gives one flexibility and freedom quite similar to that of BSD License.
In the above screen shot, you can see the TerraServer USGS Topo DRG WMS overlayed on the default NASA World Wind globe base layer. With the relief of the base layer, the topos take on a more interesting and meaningful appearance than flat 2D maps. If you might be wondering why the user interface has a tan-coloured appearance in some of the windows: that's just my selected Windows background colour. I like using this tan colour (RGB: 255,204,153 ; HEX: #FFCC99) for most of my applications (especially the background of a code editor).
I also like very much the navigation overview map in the lower left corner of the user interface. This provides a very easy navigation tool, especially if one uses Dapple on a laptop/notebook computer lacking a standard mouse.
Geosoft have started an excellent enhancement to World Wind, and I look forward to future functionality in this.