Plugins can extend WordPress to do almost anything you can imagine.
For the very latest WordPress plugins visit the official site at http://wordpress.org/plugins
Bluegoldfish has produced some plugins for various of the sites it hosts. They may not be applicable to many other sites but are available here if you would like to try them. That are all published under the terms of the GNU General Public License and so you can redistribute it and/or modify then under the terms of the that license as published by the Free Software Foundation. Visit http://www.gnu.org/licenses/gpl.html for details.