X is a heightfield engine written by codex. Included in
the download is a DOS 32-bit executable (requires a 32-bit extender,
such as cwsdpmi.exe) as well as its DJGPP source code.
The engine uses Chris Egerter's heightfield maps, which are, unfortunately,
fairly small, so you'll notice their isn't much room to move.
When neuraldk creates a new heightfield engine (possibly with 6DOF),
however, we will provide a larger, original landscape.