Exporting terrain/models

I've seen a lot of topics about importing models from blender - but are there any functions to export models to blender compatible formats?

You could write an exporter yourself to OBJ, it is an extremely simple format (text based with vertices, triangles, tex-coords, etc.) It depends how much you want to save.

Generally speaking, writing an exporter is much more easier than writing an importer. You can check the documentation for most model formats online and then simply write data which is compatible with the format.