SoftSlate Commerce has been tested with Microsoft SQL Server 2000, MySQL, PostgreSQL, and Oracle 10g. We've found that the best database to use for development is largely a matter of the particular developer's preferences and experiences.
We often use MS SQL Server 2000 ( http://www.microsoft.com/sql), which of course is only an option if you're developing on Windows. It's Developer Edition is reasonably priced. MySQL http://www.mysql.com/downloads is another favorite. It does not come with a tool to visually browse the database and make updates to it, but several options are available if that is a concern, including phpMyAdmin and the MyEclipse database browser.