WordPress.org

Akismet checks your comments against the Akismet Web service to see if they look like spam or not.

By: Matt Mullenweg, Ryan Boren, Andy Skelton, Michael Adams (mdawaffe), and others.

A very fast caching engine for WordPress that produces static html files.

By: Donncha O Caoimh, automattic, and Brandon Kraft.

BuddyPress helps you run any kind of social network on your WordPress, with member profiles, activity streams, user groups, messaging, and more.

By: John James Jacoby, Paul Gibbs, Boone Gorges, r-a-y, imath, @mercime, and others.

Your WordPress, Streamlined.

By: automattic, aduth, Alex Kirk, allendav, alternatekev, Andy Skelton, and others.

A simple and easy way to test your theme for all the latest WordPress standards and practices. A great theme development tool!

By: Samuel Wood (Otto) and Simon Prosser.

bbPress is forum software, made the WordPress way.

By: Matt Mullenweg, John James Jacoby, Jennifer M. Dodd, and Stephen Edgar.