MongoDB (https://www.mongodb.com/) is a cross-platform document store and is one of the most popular choices for NoSQL databases. It has a proprietary JSON-based API and query language. It supports MapReduce and enterprise features such as sharding, replication, and so on. It is free and open source software.
MapReduce is a data processing paradigm where a job is split into multiple parallel map tasks, with the produced output sorted and reduced into the result. This makes processing large datasets efficient and faster.