OctopusKit is described as 'A 2D game engine based on ECS and written in 100% Swift for iOS, macOS and tvOS' and is a Game Development Tool in the Development category. There are more than 25 alternatives to OctopusKit for a variety of platforms, including Windows, Mac, Linux, iPhone and Android. The best alternative is Godot Engine, which is both free and Open Source. Other great apps like OctopusKit are Unity (Freemium), LÖVE (Free, Open Source), GameMaker: Studio (Paid) and Cocos2d-x (Free, Open Source).
Godot is a fully featured, open source, MIT licensed game engine. It focuses on having great tools, and a visual oriented workflow that can export to PC, Mobile and Web platforms. It is a cross-platform 2D and 3D game engine and game development IDE.
Continuing the legacy of Mark Overmars' Animo and Game Maker, GameMaker: Studio is a proprietary game-creation system which can export to multiple platforms including Windows, HTML5, Linux, iOS, Mac and Android.
Pygame is a set of Python modules designed for writing video games. Pygame adds functionality on top of the excellent SDL library. This allows you to create fully featured games and multimedia programs in the python language.
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.