Fontspec is a package for XeLaTeX.  It provides an automatic and
unified interface to feature-rich AAT and OpenType fonts through the
NFSS in LaTeX running on XeTeX.  XeTeX (the successor to TeX-GX) is a
TeX-variant (originally for Mac OS X, but now available in the major
distributions for other platforms, too) that accepts unicode input and
can access system fonts directly.