Dungeondraft and Wonderdraft, Godot and whatnot. Share tools, mods, assets (mention the license!), etc.
Let’s take advantage of threading: post the tool up top, and any stuff for it goes in a reply. Keeps things tidy. Or creates chaos. One way to find out!
Tiled is a tilemap or tile-based level editor. It’s free and open source software licensed under the GPL-2.