Since this forum is about various engines as well, I wanted to let you know that the engine for our game's client (Eternal Lands) is open source. While it is targeted for client/server, it can be easely modified to make a single player game with it. It is a 3D isometric engine, uses OpenGL, and compiles fine on Windows, Linux and FreeBSD (an OSX port is on the way). To download it, follow the link in my sig., go to the download page, and download the source from the CVS.

Interesting - how well documented is it?

Well, the code IS commented (where necesary), but there is no documentation to it (like a manual). However, given the fact that many of our players were able to release patches, I think it's pretty easy to understand and modify it.