XeTeX (by Jonathan Kew) is a successor to the shareware TeX/GX
program. It is a Unicode-based (UTF-8) TeX implementation which
is able to make use of Mac OS X AAT (Apple Advanced Typography)
.dfonts and OpenType fonts. It uses Apple's Quartz system
(which facilitates the afore-mentioned font access) to generate
PDF output.
XeTeX requires Mac OS {X} 10.3 "Panther" or later (although there has been some success using earlier versions). It is especially oriented towards multi-lingual typography, taking advantage of Apple's keyboard input systems and system font support for various languages.
The system is able to make use of Gerben Wierda's TeX for Mac OS X setup, but a minimal set of TeX support files is also available, making an attractive proposition for those with limited bandwidth who are inclined to use Plain TeX. It can be installed using Apple's Installer, or as an i-Installer package.
The project has a web site for the user who wants more than this simple answer, and you can also sign up to a mailing list.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=xetex