Flixel Game Engine is described as 'Flixel is an open source game-making library that is completely free for personal or commercial use' and is a Game Development Tool in the Development category. There are more than 10 alternatives to Flixel Game Engine for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is LÖVE, which is both free and Open Source. Other great apps like Flixel Game Engine are Pygame (Free, Open Source), nCine (Free, Open Source), FreeBASIC (Free, Open Source) and GeeXLab (Free).
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.
FreeBASIC is a completely free, open-source, 32-bit BASIC compiler, with syntax similar to MS-QuickBASIC, that adds new features such as pointers, unsigned data types, inline assembly, object orientation, and many others.
Amulet is a Lua-based toolkit for experimenting with interactive graphics and audio. It provides a cross-platform API for drawing graphics, playing audio and responding to user input, and a command-line interpreter for running Amulet scripts.