Stellar is described as 'open-source program inspired in GameMaker for Pygame/Python development' and is a Game Development Tool in the Development category. There are four alternatives to Stellar for a variety of platforms, including Windows, Mac, Online / Web-based, FreeBSD and Haiku. The best alternative is Godot Engine, which is both free and Open Source. Other great apps like Stellar are Gamesalad (Paid), CrossBrowdy (Free, Open Source) and Invader Maker (Freemium).
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.
GameSalad Creator is an authoring tool developed by GameSalad, Inc. (formerly Gendai Games) aimed primarily at non-programmers for composing games in a drag-and-drop fashion, utilizing visual editors and a behavior-based logic system.
Make and play game levels in space invader style including run, jump, and fly mechanics. Player clears out enemies screen by screen/level by level. Lots of options to make challenging and unique enemies.