WordPress.org

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

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

bbPress is forum software, made the WordPress way.

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

Increase your traffic, view your stats, speed up your site, and protect yourself from hackers with Jetpack.

By: Automattic, Adam Heckler, Andrew Duthie, Alex Kirk, allendav, alternatekev, and others.

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.

BuddyPress helps you build any type of community website using 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.

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.