As promised earlier this year, Pivotal released the code for Greenplum Database into Open Source.
Greenplum Database is based on PostgreSQL (was forked from PG 8.2), and features a massive parallel processing system (MPP) to run SQL queries on very large data sets. The code base is licensed under the Apache 2.0 license, and available on GitHub. You can fork the project from there, or submit patches and new features.