screenshot of Godot v3.0, rapidly emerging as a popular free and open source community alternative to Unity and Unreal
2D level editors
If your engine already has a built-in 2D level editor, then use that. But if you're using a homemade engine or web-based framework, you'll need a standalone 2D level editor.
Unlike the fragmented 3D editor ecosystem, all standalone 2D level editors are open-source, stable, and engine-agnostic with easily parsed JSON file formats. Here we generally recommend Tiled, with its many features and widespread engine support.
screenshot of Tiled, a free open source standalone 2D tile-based level editor
3D art tools
In most cases, we don't recommend using 3D modeling tools to build levels. That said, all these tools basically do the same thing, and you should use whatever tools you like using.
We generally recommend Blender, free open source software that now rivals commercial tools. Older artists often prefer Maya or 3DS Max because they already learned it + industry pipelines are tightly coupled. But let's be clear -- Blender is basically the future, and Autodesk's days are numbered.