There are two primary companies that provide commercial PostgreSQL products. These companies have re-branded PostgreSQL to fit into their own product lines. They are Great Bridge, LLC, and Red Hat, Inc.
The Great Bridge version of PostgreSQL is at 7.1, and is currently the same version of PostgreSQL distributed from the community. Like most Open Source companies, Great Bridge specializes in consulting, training and support for their product. If you purchase Great Bridge PostgreSQL, you will receive 30 days of installation support in addition to the software. This product is available for both Linux and Solaris.
The Red Hat version of PostgreSQL is called Red Hat Database, and is a very new product to the database market. Red Hat Database is based on the currently distributed Open Source version, PostgreSQL 7.1. Red Hat Database currently supports Linux only, and preferably Red Hat Linux 7.1.