Defold is a free and open-source game development framework for Windows, Mac and Linux platforms. It's renowned for its intuitive design, powerful features, and extensive community support. Its focus on rapid iteration and collaboration makes it an ideal choice for teams of all sizes, from solo developers to large studios.

Gideros is a free, open-source and cross-platform game engine, developed by Gideros Mobile from Turkey in 2011. It allows developers to quickly and easily create various types of applications and games for both mobile and desktop platforms, including iOS, Android, Windows, and Mac.

GDevelop is a free, open-source and cross-platform 2D game engine designed to be used by everyone, originally created by Florian Rival (4ian) from United Kingdom in 2008, using C++ and JavaScript. We can use it to create any type of 2D game for Windows, Mac, Linux, iOS and Android platforms, as well as HTML5 Web browsers, Steam platform and Universal Windows Platform.

Game Creator is a complete set of professional, powerful and intuitive tools that can help you kickstart your game development in a matter of minutes. To some extent, it allows you to quickly create any kind of game with ease. It is compatible with Unity all editions, but requires Unity 2019.4 or newer versions.

To learn 3D game development with limited pockets, I guess you've tried many classic, free, open-source game development engines like TV3D (Truevision3D), OGRE and Irrlicht Engine. By contrast, the pros of TV3D are easy to learn and get started with; its cons are that it was built based on the ActiveX control and requires the RunTime DLL to be registered before running; OGRE is better than Irrlicht in that it has a capable and vigorous engine framework, and supports for extending new functionality in the form of plug-ins (this is better than Irrlicht); but its downside is that it's not easy to get started with.