An old hobby project from 2006 written while attending college. 3D terrain generation, Lua scripting integration, third-party plugin framework for extension. Developed back from when libraries for supporting this stuff weren't common, so it's all hand-rolled (including the DirectX graphics).
Definitely not state-of-the-art, but probably my favorite thing I wrote while in college. And it was done as a hobby over six months, alongside a (more than) full courseload. :)
Raw binaries can be downloaded from here. Find and run Terraingine.exe on a Windows system (ignore the drag-and-drop failures).