There’s a cool set of blog postings over at Twenty-Sided where the author describes the process he went through to build a decently looking 3d landscape rendering engine. His description is a little to high level in my opinion, but he does provide full source code at the end for those folks who want to dive in deeper. All in all it’s a pretty facinating set of posts for anyone interested in 3d game engines.
The author also links to the Virtual Terrain Project. I’d never heard of it before, but it seems to be a compilation of almost everything you could ever want for working on a 3d terrain engine (for a game or otherwise.
Also somewhat related is this site that has a series of OpenGL tutorials.