The original Overlap2D
Overlap2D is a 2D level and UI editor with an engine agnostic philosophy for game development. Made to separate coding from content, it enables developer to create rich content using images, animations, particle effects, light system, physics and complex grouped items.
The project is hosted on github, but seems to be abandoned.
HyperLap2D?
HyperLap2D strives to be a modern replacement for Overlap2D. It is made to be a powerful, platform-independent, visual editor for complex 2D worlds and scenes.
VisEditor (discontinued)
VisRuntime is no longer maintained. It was an open source 2D game level editor with a libGDX runtime.
You can find more information at https://kotcrab.com/blog/2017/03/26/visui-130-released-viseditor-deprecated/.