Also known as STEdit, this is a terrain creation utility created by Soleil Lapierre. Latest version is v1.7, made in 1994.
This utility is only a terrain creation tool with the view of the whole terrain at once (30x30). The main utility of this program is a very good feature that allow you to create a full terrain (valid by game engine, with no bug) automatically in one click !
This program supports mouse which make it really user-friendly compared to other tools.
Sadly, this program doesn't allow you to load another terrain once you started to work with one, you have to quit the program and start it again when you want to change terrain, which is boring.