There are a few free terrain apps that can save a DEM to triangles, though perhaps not to STL directly.

Mesh to Single Nurbs http://www.resurf3d.com/ will take an STL or XYZ format file and build a nurbs surface out of it. I have gotten mixed results, but overall I think it is better than ZSurf. While its not free, it is not very expensive. It does require some experimentation to get the best results, and because extends the patches it produces beyond the existing geometry, you need to trim it up in a nurbs modeling app.

If you have Rhino, there's a RhinoResuf, a plugin by the same company that offers more options in producing nurbs surfaces from meshes, including trimming the nurbs to the mesh.

Does it need to be done with NURBS? If I'm not mistaken, you can download Bryce for free. It will export in 3DS, OBJ, DXF, and DAE formats.

http://www.daz3d.com/i/software/bryce/download?

Steve S

it's pretty cool that they're offering Bryce for free now. the only problem is that i can't import it into MOI so i can turn it into a game tile that i can create with my CNC machine. good idea though :)

Here is a bryce terrain converted to an iges by the previously posted software and loaded into MoI...