Plugins are tools to extend the functionality of WordPress. This article contains a list of plugins by category, and links to other plugin repositories. The core of WordPress is designed to be lean, to maximize flexibility and minimize code bloat. Plugins offer custom functions and features so that each user can tailor their site to their specific needs.
For instructions and information on downloading, installing, upgrading, troubleshooting, and managing your WordPress Plugins, see Managing Plugins. If you want to develop your own plugin, there is a comprehensive list of resources in Plugin Resources.