Impactjs is a 2D HTML5 game engine with a built in level editor called Weltmeister. Previously it was commercial software with a $99 price tag. Earlier today the author made the following tweet:
The engine is now available under the MIT open source license. It’s available on Github right here. Unfortunately the editor has some PHP dependencies, making it somewhat tricky to get up and running without a full blown PHP install.
It’s been several years since Impact was updated, so unless a community forms around this libraries open sourcing, I see little reason to recommend it.