747b0392__5_1___Postgres_OnLine_Journal__Microsoft_SQL_Server_2008
[Feature] Licensing [MySQL 5.1] GPL Open Source, Commercial. Now owned by Oracle, and some of us are still wondering how Oracle will make hay out of its new found treasure. Will they use to upsell. Monty Widenius has an interesting blog entry explaining the MySQL dual licensing and how it has changed. [PostgreSQL 8.4] BSD Open Source. Its the freest of all and many argue about it. You can fork it all you want without giving back and make your own commercial derivative. Though ironically there are not all that many forks. Netezza forked it initially (though its unclear if any PostgreSQL code lives on in their product), GreenPlum forked it, EnterpriseDb forked to give Oracle features, but to their credit gives back a lot of fixes to the community. [Microsoft SQL Server 2008] 
Commercial - Closed Source, Various levels of features based on version, Free Crippleware (4 GB limit but free for embedding inclusion)
Cross Compare of PostgreSQL 8.4, SQL Server 2008, MySQL 5.1 - Postgres OnLine Journal
Microsoft SQL Server 2008