Plugins are tools to extend the functionality of WordPress. This tag should not be used to mark requests for plugin recommendations, which are off-topic per the FAQ.
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.
WordPress Plugins are available from several sources. The "official" sources for WordPress Plugins are:
Not all WordPress Plugins make it into the above repositories. Try searching the web for "WordPress Plugin" and the keywords for the type of functionality you are seeking.
The following plugins are included with the WordPress core distribution:
Akismet checks your comments against the Akismet web service to see if they look like spam or not. You can review the spam it catches under "Manage" and it automatically deletes old spam after 15 days. (Be careful - Akismet is free only for personal use.)
This is not just a plugin, it symbolizes the hope and enthusiasm of an entire generation summed up in two words sung most famously by Louis Armstrong. Hello, Dolly. This is, by the way, the world's first official WordPress Plugin. When enabled you will randomly see a lyric from "Hello, Dolly" in the upper right of your Administration Panels on every page but the Plugins panel.