Apps similar to Flax Engine include Godot Engine, which is free and open source. Other options are Unity, Unreal Engine, GDevelop and Scratch. There's no shortage of Game Development Tools like Flax Engine, with more than 100 alternatives for Windows, Mac, Linux, the web and iPhone.
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.
Verge3D is a powerful and intuitive toolkit that allows Blender, 3ds Max or Maya users to create immersive web-based experiences. Verge3D can be used to build interactive animations, product configurators, engaging presentations of any kind, online stores, explainers, e-learning...
Russia
One framework for creating powerful cross-platform games. The spiritual successor to XNA with 1000's of titles shipped across desktop, mobile, and console platforms. MonoGame is a fully managed .NET open source game framework without any black boxes.
Phaser is a fun, free and fast 2D game framework for making HTML5 games for desktop and mobile web browsers, supporting Canvas and WebGL rendering.
United Kingdom
Adventure Game Studio (AGS) - is the IDE and the engine meant for creating and running videogames of adventure (aka "quest") genre. It has potential, although limited, support for other genres as well.
United Kingdom
Browser-based 2D game creation engine using a visual event-driven system, integrates JavaScript for advanced development, requires no installation or prior programming, supports exporting to HTML5, mobile platforms, Xbox One, and enables real-time editing on any device.
United Kingdom
United States
3D game engine that uses web technologies. Perfect for those who already know TypeScript and want to collaborate with designers and artists to create 3D experiences.
Sweden
EU
ENIGMA, the Extensible Non-Interpreted Game Maker Augmentation, is an open source cross-platform game development environment derived from that of the popular software GameMaker Legacy.
Create anything you can imagine with Roblox's free and immersive creation engine. Roblox Studio lets you create anything and release with one click to smartphones, tablets, desktops, consoles, and virtual reality devices.
United States
Kodu lets kids create games on the PC and XBox via a simple visual programming language. Kodu can be used to teach creativity, problem solving, storytelling, as well as programming. Anyone can use Kodu to make a game, young children as well as adults with no design or...
United States
