MySQL: all their base are belong to others. Oracle buys Sleepycat (BDB) Oracle buys Innobase (InnoDB) IBM acquire SolidDB Unless you are running MyISAM (stop reading and switch now before you lose data)... the backend you run in MySQL has a product roadmap controlled by a competitor. Think about that. To be clear, I think this does not lessen MySQL's value today. What it does is make me entirely unsure of MySQL's future value.
So, there are a lot of databases out there. I’ve used more than a handful. Commercial, Proprietary, Free, Open, etc. The two databases I’ve used most are Oracle and MySQL and let me say that I like them both immensely. However, recently we considered putting database support into a commercial, proprietary application and MySQL concerned and confused me. Recently, SPFv1 has been popular – likely due to the recent publication of SPF records by Internet giant AOL.