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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
I use tectonics.js. It’s a web-based, realistic tectonics simulator. It doesn’t give you a ton of control (except over resolution) but it’s much less work because it’s entirely randomized and you can let it run forever until you get shapes you like. The simulator will also overlay things like wind speed and direction and climate (although I’m not as convinced that those are entirely accurate—it accounts for elevation but I can’t remember if it has a humidity variable). It doesn’t do rivers either, but it gives you a topographic map so you can draw them in fairly easily. BTW—a word of warning, it allows you to export as a 2D map but the projection is a little weird so you have to resize and crop it before you upload the image file into another app to edit (which you’ll want to do to add rivers and polish the appearance a bit). tectonics.js