Learn about the Common Gateway Interface
Run your own guestbook! This sends mail to your box (in a delineated form) then posts a thank-you note to your viewer.
You don't want to attach to someone else's counter. Use your own! Grab this, put it on your page, and go nuts.
This acts as a newsgroup by posting messages people write to the guestbook!
You may need to know this to get your CGIs working.
Build a discussion board that runs just like the Goodies boards.