PostgreSQL is described as 'powerful, open source object-relational database system. It has more than 15 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness' and is a very popular relational database in the development category. There are more than 50 alternatives to PostgreSQL for a variety of platforms, including Linux, Windows, Mac, BSD and Online / Web-based. The best alternative is SQLite, which is both free and Open Source. Other great apps like PostgreSQL are MongoDB, MySQL Community Edition, MariaDB and Microsoft SQL Server.
SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world. The source code for SQLite is in the public domain.
SQL Server delivers on Microsoft’s Data Platform vision by helping your organization manage your data by enabling you to store data from structured, semi-structured, and unstructured documents within the database.
Apache CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP / JSON API. It includes incremental map / reduce mechanics for queries and keeps data safe with an append-only datastore.
Apache Cassandra has become the leading NoSQL platform driving many of today's modern business applications by offering continuous availability, high scalability and performance, strong security, and operational simplicity while lowering the overall cost of ownership.
Firebird is a relational database offering many ANSI SQL-92 features that runs on Linux, Windows, and a variety of Unix platforms. Firebird offers excellent concurrency, high performance, and powerful language support for stored procedures and triggers.
Cockroach is a distributed key/value datastore which supports ACID transactional semantics and versioned values as first-class features. The primary design goal is global consistency and survivability, hence the name.