Developer.com just published their 2008 Product of the Year awards and the audience voted. For Best Database Tool or Add-in, the nominees were: Hibernate, JavaDB, MySQL, Oracle Database 10g, and PostgreSQL.
Here is the quote from Developer.com:
And the winner is PostgreSQL. This one surprised us and taught us something about our audience base. We need to do a better job of covering this topic! PostgreSQL.org touts this product as "The world's most advanced open source database" and after seeing the votes, they may just be correct.