Web Site: http://www.macports.org

OS X is an UNIX OS.  The Aqua GUI is nice an all but in order to get the full potential of OS X, you should know how to work with the Terminal(xterm) and scripts.  Other free *NIXes have developed a nice package system and it is MacPorts that does package managements on OS X.

Requirement:
You need Apple Xcode Development Tools.  Get it from AppStore(Link):

It is free!!!  After it is installed, you need to accept Xcode EULA by either running xcode or entering the following command in the Terminal:

% xcodebuild -license

Go to Applications then double click on the Xcode so it gets installed on the system.

Now the Command Line Tools needs to be installed.  Go to Xcode->Preferences->Downloads:

Click on the “Install” button next the the Command Line Tools.

Now, MacPorts can be installed via .pkg/.dmg and the links are:
Mountain Lion: https://distfiles.macports.org/MacPorts/MacPorts-2.1.2-10.8-MountainLion.pkg
Lion: https://distfiles.macports.org/MacPorts/MacPorts-2.1.2-10.7-Lion.pkg
Snow Leopard: https://distfiles.macports.org/MacPorts/MacPorts-2.1.2-10.6-SnowLeopard.pkg
Leopard: https://distfiles.macports.org/MacPorts/MacPorts-2.1.2-10.5-Leopard.dmg

After downloading the .pkg/.dmg, simply double-click on it to get it installed.

Self update: in a Terminal type the following:

% sudo port -v selfupdate