PostgreSQL is great, 8.4 looks super

PostgreSQL is entirely free whereas there was a fee if you bundled MySQL with an app. PostgreSQL has a very long development history and is stable. It is ACID compliant from the ground up and adheres better to SQL standards than most. MySQL gains speed at the expense of reliability and there is some doubt as to whether it is really faster in a high traffic environment. MySQL may have a broader user base but there are a lot of folks who are quietly using PostgreSQL. I know of a company in Denver that uses PostgreSQL in extremely high traffic e-mail applications. I also just saw that MusicBrainz uses it. The big problem early on was that PostgreSQL wasn't available for Windows, but that has been addressed and I think it is now, deservedly, taking off.