MonoGame

Cross-platform, open source .NET framework. Sponsored by Microsoft, Mono is an open-source implementation of Microsoft's .NET Framework as part of the .NET Foundation and based on the ECMA standards for C# and the common language runtime. A growing family of solutions and an active and enthusiastic contributing community is helping position Mono to become the leading choice for the development of cross-platform applications. The latest Mono release is waiting for you. We cover everything you need to know, from configuring Mono to how the internals are implemented. Our documentation is open source too, so you can help us improve it. As an open-source project, we love getting contributions from the community. File a bug report, add new code or chat with the developers. Mono is a software platform designed to allow developers to easily create cross-platform applications.

Defold is a free and open source game engine designed for high-performance cross-platform game development. It comes fully featured out of the box, requiring no setup or configuration, allowing developers to simply download the editor and start creating. The engine includes a visual editor, code editor, Lua scripting, Lua debugger, scene editor, particle editor, and tilemap editor, and supports both 2D and 3D game development. With a single codebase and no need for external tools, Defold enables publishing to major platforms such as PlayStation5, PlayStation4, Nintendo Switch, Android, iOS, macOS, Linux, Windows, Steam, HTML5, and Facebook. It offers a component-based system for building games, allowing the use of basic building blocks to create complex behavior, and supports writing game logic using Lua. Developers can add new functionality from the asset portal, set up their own local build environment, and write native code to extend the engine.