PostgreSQL is a robust relational database system with over 25 years of active development that runs on all major operating systems. It is fully ACID compliant, and has full support for foreign keys, joins, views, triggers, and stored procedures (in multiple languages). It includes most SQL92 and SQL99 data types, including INTEGER, NUMERIC, BOOLEAN, CHAR, VARCHAR, DATE, INTERVAL, and TIMESTAMP. It also supports storage of binary large objects, including pictures, sounds, or video. It has native programming interfaces for C/C++, Java, .Net, Perl, Python, Ruby, Tcl, and ODBC, among others, and exceptional documentation.
Berkeley DB (libdb) is a programmatic toolkit that provides embedded database support for both traditional and client/server applications. It includes b+tree, queue, extended linear hashing, fixed, and variable-length record access methods, transactions, locking, logging, shared memory caching, database recovery, and replication for highly available systems. DB supports C, C++, C#, Java, PHP, and Perl APIs. It supports key-value pair (NoSQL), SQL, and Java Object formatted data. It is available for a wide variety of Unix platforms as well as QNX, Android, Mac OS X, and several varieties of Windows.
KInterbasDB is a Python extension package that implements Python Database API 2.0-compliant support for the open source relational database Firebird and some versions of its proprietary cousin Borland Interbase. In addition to the minimal feature set of the standard Python DB API, KInterbasDB also exposes nearly the entire native client API of the database engine.
MariaDB is a database server that offers drop-in replacement functionality for MySQL. It is built by some of the original authors of MySQL, with assistance from the broader community. In addition to the core functionality of MySQL, it offers a rich set of feature enhancements including alternate storage engines, server optimizations, and patches.
Qddb is powerful database creation software. Using Qddb, you can quickly and easily create database applications. nxqddb allows you to create a complete database application in less than five minutes, and Hx lets you manage database information over the web through a CGI interface. Larger applications can also be developed using Fx, a powerful TCL/Tk toolkit.
HailDB is a relational database in shared library form that can be embedded into applications. It is not a SQL database, although you can use this library as the storage backend for a SQL database. It features a clean and simple API and is a continuation of the Embedded InnoDB project. With the addition of the HailDB plugin to Drizzle, you get a full SQL interface.
Tyserv is a DBMS for UNIX-like systems. It has isam-like access, transaction control, rollback, commit, exclusive lock, access control by client IP address, and user authentication. If you like old but simple database programming environments like COBOL+VSAM(KSDS) or C+C-ISAM, you will like Tyserv.