Search through the NEWS file for "X Resources". A fairly comprehensive list is given after it.
In addition, an app-defaults file is supplied, `etc/Emacs.ad', listing the defaults. The file `etc/sample.Xdefaults' gives a set of defaults that you might consider. It is essentially the same as `etc/Emacs.ad' but some entries are slightly altered. Be careful about installing the contents of this file into your `.Xdefaults' (.Xresources) file if you use FSF GNU Emacs under X11 as well.