OpenNERO is described as 'open source software platform designed for research and education in Artificial Intelligence'. There are three alternatives to OpenNERO for Linux, Mac, Self-Hosted solutions and Windows. The best alternative is OpenAI Universe, which is both free and Open Source. Other great apps like OpenNERO are SerpentAI (Free, Open Source) and ELF (Free, Open Source).
Serpent.AI is a simple yet powerful, novel framework to assist developers in the creation of game agents. Turn ANY video game you own into a sandbox environment ripe for experimentation, all with familiar Python code.