PostgreSQL is an Enterprise ready object-relational database management system (ORDBMS) used by many companies including Yahoo! and Skype. Completely open source and cross platform, it is a viable alternative to the Oracle database. PostgreSQL is designed for high volume environments and as such is highly extendable and customizable. Like Oracle, PostgreSQL has it's own scripting language (PL/pgSQL) and support for most of the other scripting languages and compiled languages including C, C++ and Java.
PostgreSQL supports many data types natively including text and composite arrays, XML and IPv4 and IPv6 addresses, and binary large objects (blobs). It also supports user defined data objects such as Domains, Functions and Indexes. Other features include Multi-Version Concurrency Control (MVCC), online backups, point in time recovery and asynchronous replication.
In the past, PostgreSQL was considered to be difficult to configure, slow to respond and not very user friendly. However, with latest versions, those issues have, in the large part, been resolved. The documentation is excellent, and there is community support through mailing lists and IRC. Commercial support is available through third party vendors.