Forum Moderators: open
As far as general purpose directories I am not sure that any, other than DMOZ and Yahoo, are worth much effort. Zeal used to be valuable as a way of getting into the MSN directory results, but it is either on its way out or already out (not sure, I do not follow that area) and Google does not spider its directory.
Note that the value of Yahoo directory as a source of referrals has gone down since Yahoo changed their search strategy many months ago. I believe that many (most?) people view a Yahoo directory listing merely as a source of PR.