As of March 2014, 79% of all Debian packages are available for Debian GNU/Hurd. Of course, testing and bug fixing is welcome, as we have obviously not tested all of them.
Generally, packages that aren't Linux-specific (see Packages That Won't Be Ported) are expected to work on GNU/Hurd too. Notably, X.Org, GNOME, KDE, Firefox work. See the guidelines document for some common build problems and their solutions.