MongoDB

MariaDB is an open-source relational database management system that is a fork of MySQL. It's designed to be a drop-in replacement for MySQL with more features, new storage engines, and improved performance. MariaDB offers features like advanced clustering with Galera Cluster, more storage engines, and better performance optimizations. Its commitment to remaining open-source and its compatibility with MySQL make it an attractive option for organizations looking for a powerful, community-driven database solution.

Redis is an open-source, in-memory data structure store that can be used as a database, cache, message broker, and queue. It supports various data structures such as strings, hashes, lists, sets, and more. Redis is known for its high performance, flexibility, and support for atomic operations. Its ability to persist data to disk and replicate data across multiple servers makes it suitable for a wide range of use cases, from simple caching to complex real-time applications. Redis's speed and versatility make it popular among developers for building high-performance, scalable applications.

PostgreSQL, also known as Postgres, is a powerful, open-source object-relational database system. It emphasizes extensibility and SQL compliance, offering advanced features like complex queries, foreign keys, triggers, and transactional integrity. PostgreSQL is known for its reliability, data integrity, and ability to handle complex workloads. Its support for both relational and non-relational querying makes it versatile for various applications. PostgreSQL's robust feature set and active community make it popular among developers and organizations requiring a sophisticated, scalable database solution.

Percona server is an opensource, fully compatible, enhanced drop-in replacement for MySQL, providing superior performance, scalability, and instrumentation.

Solr is an open-source enterprise-search platform, written in Java.