GDevelop is an open-source game making software designed to be used by everyone. Any kind of 2D games can be created with it and no programming skills are required to get started. Logic of games are made using visual events.
The RPG Maker software series is a set of game creation suites aimed at easy 2D role playing game creation. With an emphasis on JRPG gameplay mechanics, the suites typically include map, tile and sprite editors, character, encounter and dialog tools, among countless other...
cocos2d-x is a cross platform open source free 2D game engine for mobile gamedev, that is fast and stable, easy to learn and use.
Flax Engine is claimed the most agile game engine. It's open source, cross-platform and it lets you write scripts in C++ and C# seamlessly. Also focused on lightweight development. A true game changer.