Rails (in the form of Active Record) ships with a Ruby-based MySQL adapter, but it's much advised to get the C-based one as it's considerably faster (runs the unit tests in twice the speed). You also need to get a database driver if you want to use [[SQLite]] or [[PostgreSQL]] (for which you can also use a slower, but Gems-installable pure Ruby implementation). These are the ones you want:

* "MySQL-ruby":http://www.tmtm.org/en/mysql/ruby/ by Tomita Masahiro
* "SQLite-ruby":http://sqlite-ruby.rubyforge.org/ by Jamis Buck
* "Postgresql-ruby":http://www.postgresql.jp/interfaces/ruby/ by Yukihiro Matsumoto and Eiji Matsumoto

h2. Supported DBs

* [[MySQL]]
* [[PostgreSQL]]
* [[SQLite]]
* [[SQL Server]]
* [[IBM DB2]]
* [[Oracle]]
* LDAP - Not exactly a DB driver, but ActiveLDAP can be used with Rails.

h2. Requested adapters

These are adapters that Rails does not yet support, but people would like to see:

* [[ODBC]]
* [[JET/OLEDB]]
* [[Sybase ASE]]
* [[Firebird/Interbase]]
* [[SQL Server (without Win32OLE dependancy)]]
* [[Berkeley DB/DB XML]]
* [[Informix]]
* [[eXist]]
* [[XBase (DBF/CDX driver)]]
* [[Lotus Notes/Domino]]
* [[OpenBase]]