A lot of people are saying MySQL but sqlite is already installed and the Ruby on Rails that ships with Mac OS X 10.5 uses it as default. Also if you do any developing in Python then version 2.5, which ships with Mac OS X 10.5, has sqlite3 support.
I use to use MySQL for development along with CocoaMySQL for graphical viewing but honestly I think sqlite is fine for just developing something in Django or Rails.
If you are a visual person then you should check out sqlitebrowser. It’s a bit clunky but it works fine—just don’t try to open a db file that is already in use.
As for me, I use Textmate, CSSEdit, Pixelmator, and Transmit. I’m also big in to version control and use SCPlugin for Subversion viewing in the Finder and also a textmate svn bundle.