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!
https://kenney.nl has thousands of free assets (most if not all of them are CC0)
https://opengameart.org is a site to share and find any kind of free to use asset (various licenses)