PostgreSQL is the default database on Mac OS X Server as of version 10.7. The standard version of Mac OS X includes only the PostgreSQL commandline client utilities.
Mac OS X Server 10.7 ships with PostgreSQL 9.0. Minor updates are provided by Apple, but not necessarily right after a new PostgreSQL minor release.
There are several other installers available for PostgreSQL on Mac OS X, which is the recommended way to install.
An installer is available for Mac OS X that includes PostgreSQL, pgAdmin and the StackBuilder utility for installation of additional packages. For PostgreSQL 9.0 and 9.1, Mac OS X 10.5 and above are supported, on 32 and 64-bit Intel CPUs, and PostgreSQL 9.2 and later support Mac OS X 10.6 and above on 32 and 64-bit Intel CPUs.
Download the installer from EnterpriseDB for all supported versions.
Postgres.app is a simple, native Mac OS X app that runs in the menubar without the need of an installer. Open the app, and you have a PostgreSQL server ready and awaiting new connections. Close the app, and the server shuts down.
PostgreSQL packages are available for Mac OS X from the Fink Project. Please see the Fink documentation for information on how to install packages.
A list of PostgreSQL packages can be found using the package search tool on the Fink website.
PostgreSQL packages are also available for Mac OS X from the MacPorts Project. Please see the MacPorts documentation for information on how to install ports.
A list of PostgreSQL packages can be found using the portfiles search tool on the MacPorts website.
PostgreSQL can also be installed on Mac OS X using Homebrew. Please see the Homewbrew documentation for information on how to install packages.
A list of PostgreSQL packages can be found using the Braumeister search tool.