I implemented a terrain rendering engine based on the geometry clipmapping algorithm using C# and OpenTK. It streams terrain data from the NASA World Wind terrain servers and imagery from Esri's REST map services, so the terrain and imagery cover most of the world and are quite good. Thanks to OpenTK, it runs under both Windows and Linux.
The source code is available on Sourceforge: http://sourceforge.net/projects/miniglobe/ and is available under the permissive MIT license. Instructions for building it, as well as a ZIP file of the source code, are available here: http://www.virtualglobebook.com/code.html. You'll need an OpenGL 3.3 compatible GPU to run it.
There's a lot of room for improvement, but I think it's already pretty cool. I hope you'll find it interesting and useful.