As of version 21.5b24 XEmacs has support for Xft fonts included (but it must be selected at build time). The release notes at have all the details. Additional configuration suggestions available from CFruhwirth’s Weblog.

GNU Emacs

Support for this is in GNU Emacs version 23.1. See XftGnuEmacs. Most X systems will have sub-pixel antialiasing configured by default, but see this HOWTO if you have trouble.

AquamacsEmacs (on Mac OS X) comes configured to use a range of scalable fonts with antialiasing, which are usable out-of-the box.