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.
A Linux version has been announced (2006); that version of course can’t use AAT fonts, and relies on OpenType fonts alone.
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