Check out the open source 2D level editor 'LEd' from the former lead on Dead Cells

LEd is a very interesting project to follow for both game developers and people looking to get into it, a new free level editor with a firm aim on being user friendly.

What makes it quite interesting, is that it's built by a developer who has been part of a successful indie team. Sébastien Bénard was the lead on Dead Cells, one of my absolute favourite pixel-art platformers and it's a very popular game. LEd is based on their experience creating games, and their goal is to create a modern level editor for everyone.

Feature Highlight:

• Easy to use: modern UI with a strong focus on ease-of-use and quality-of-life features.
• Universal and agnostic: compatible with all languages (not only Haxe) and game frameworks in the world
• JSON: easy to parse file format for any game-engine out there (I promise it’s actually really easy). Haxe isn’t required.
• Customizable layers: Integer grid layers, Tile layers and Entity layers support
• Auto-layers: paint your collision map and see the grass, textures and all the small details being drawn automatically!
• Entities: fully customizable Entity with custom properties (ex: you can have a “Mob” entity, with a “hitPoints” field, which is an Integer limited to [0,10] bounds).
• Enums: you can define an enumeration (ex: an “ItemType” enum with “Money”, “Ammo”, “Gun” values) and use this enum in your entity custom fields.
• External enums: enums can be imported and synced directly from Haxe source code files (HX file)!
• HTML5: LEd is built around modern web standards.
• LEd loves Haxe: a powerful Haxe API which gives you access to fully typed values from your levels. It avoids mistakes like mistyping, renaming or removals: you see errors during compilation, not at runtime. 

After I saw the description mentioned it was open source but it didn't have a license, I opened a ticket with the developer who very promptly sorted it and it's now under the MIT license! That might be the most pleasant interaction I've had when querying that sort of thing. Additionally, they have an open ticket tagged as "help wanted" to sort a proper Linux build out so if you know Haxe it might be a fun project for you to help with. I've already jumped in to let them know of some issues that are currently being worked through.

You can see the website here and find the code on GitHub.

If you're after more like this, do check out Tilekit too.