Lars Magne Ingebrigtsen <larsi(a)gnus.org> writes in xemacs-beta(a)xemacs.org:
 I've taken to using flyspell under Emacs 20, and I really like
it.
 (It highlights things I've misspelled and stuff.)  Has it been ported
 to XEmacs (if any porting is needed), and could a package be made for
 XEmacs 21? 
Unless it has been broken by Emacs integration, it looks like it's
already ported to XEmacs.  I've added it to the text-modes packages.