Most Rails tutorials use MySQL as their canonical database. Locomotive includes SQLite, but not MySQL.
The consensus seems to be that installing MySQL via the official Mac OS X MySQL installer is the best way. It installs in /usr/local/ and includes a System Preferences pane for easy startup and shutdown.
MySQL installs with 4 accounts (including 2 root accounts) that have no password. While the default install only allows connections from localhost, it’s still a good idea to set the passwords to something. MySQL AB has a help page on Securing the Initial MySQL Accounts.
For manipulating MySQL databases directly, the open source CocoaMySQL seems nice. I used it to secure the initial MySQL accounts: Tools->Edit User.
