PostgreSQL Plus Advanced Server/EDB Server: EnterpriseDB's fork of PostgreSQL which has Oracle compatibility and some other tools (like xDB replication). Sometimes includes features from future versions of PostgreSQL
CitusDB: latest/greatest big data scale-out version of PostgreSQL. Soon to be open-source.
Aster: prior generation of PostgreSQL MPP and Map/Reduce scale-out.
Hadapt: proprietary PostgreSQL+Hadoop fusion. Based on HadoopDB, which was open source.
Paraccel: Column-oriented in-memory cluster database built from PostgreSQL 8.2.
RedShift: Amazon's fork of Paraccel, available only on AWS as a service.
Vertica: Another PostgreSQL-based column store. Unclear on how much PostgreSQL code it uses, but uses a version of the PostgreSQL protocol and psql client.
Vertica was written from scratch in C++. Maybe gram.y was used from Pg. But the client and SQL are strongly inspirited by Postgres. vsql is not psql probably, because psql is better. Last three years is not Vertica protocol compatible with Postgres. I don't know a reason why - I am expecting some strange marketing
Pavel
FAST: Fujitsu's spin of PostgreSQL, optimized for high performance on high-end hardware.
-- -- Josh Berkus Red Hat OSAS (any opinions are my own)