MM8LevelEditor 3.47 is the most stable editor.
This is probably the editor you are looking for, even though it is no longer being developed.
MMLevelEditor is the new editor being developed. It does not yet support editing everything that the MM8LevelEditor supported. However, it does allow you to modify outdoor maps for MM6, and possibly MM7. Use at your own risk.
UnLod is the original viewer upon which MM8LevelEditor was based.