MongoDB is described as 'Document database with the scalability and flexibility that you want with the querying and indexing that you need' and is a very popular relational database in the development category. There are more than 100 alternatives to MongoDB for a variety of platforms, including Linux, Windows, Mac, Self-Hosted and Web-based apps. The best MongoDB alternative is PostgreSQL, which is both free and Open Source. Other great apps like MongoDB are SQLite, MySQL Community Edition, MariaDB and Redis.