EnterpriseDB’s Postgres Plus Advanced Server. It is really a nice product which has been launched by company.
I have worked on it and found suitable and replacement of the Oracle.
With Learning of Postgres Plus Advanced Server, people also learn about the PostgreSQL, world’s most advanced Open source database.
It has almost all feature, which is require for Oracle Developers.
Here is a link, which talks about PostgreSQL and Postgres Plus
And following is a link, which has description of Oracle Compatibility provided by EnterpriseDB in Postgres Plus Advanced Server: