The dominant document database. Server Side Public License since 2018; the reference flexible-schema database for many web apps.
From Wikipedia
MongoDB is a source-available, cross-platform, document-oriented database program. Classified as a NoSQL database product, MongoDB uses JSON-like documents with optional schemas. Released in February 2009 by 10gen, it supports features like sharding, replication, and ACID transactions. MongoDB Atlas, its managed cloud service, operates on AWS, Google Cloud Platform, and Microsoft Azure. Current versions are licensed under the Server Side Public License (SSPL). MongoDB is a member of the MACH Alliance.