EnterpriseDB (EDB) is a database platform that is based on PostgreSQL. Rather than forking the open-source PostgreSQL code, they've developed their product as a collection of proprietary (non-open) extensions on top of PostgreSQL.
Compatibility with Oracle
EnterpriseDB recently claimed that they are (drop-in) compatible with Oracle applications, but at a much reduced price. However, find out just how compatible they really are before committing any applications to it. EnterpriseDB also haven't publicly announced with what Oracle products and what versions of those products they are compatible with.
EDB also offers a PL/SQL-compatible extension language.
- PostgreSQL - open-source database