Mac OS X includes the tool /usr/sbin/installer, which is a command-line version of the Installer application.
By passing it command line parameters, you can install anything that's in a .pkg or .mpkg package. This could be useful for login scripts or other automatic methods of deploying software.
[Editor's note: The Read Me explains things pretty well, and includes a number of examples.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021211060029350