sending fax messages through an ibook

It is built in to OS X, there is an option under Fax and Printers in System Preferences to enable faxing and show the status in the menu bar for sending and recieving.

You can fax by 'printing to a fax'. Go into the app you want to fax from, ie a Word document, etc, go to Print - depending on the OS, you may have the fax option there, or you have to click the PDF drop down, and choose Fax PDF - you can then set all the options for faxing.

I don't know about built in software but there is definitely software, shareware and freeware available for OSX that let's you send a fax.

I've used Faxcenter before:
http://versiontracker.com/php/search.php?mode=basic&action=search&str=fax&plt%5B %5D=macosx&x=0&y=0

And J2 global communications www.j2.com offers this software and service for an annual fee I believe

Faxing software is built in, no extra software needed to send or receive.

The fax option is under print-pdf-fax pdf.