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

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

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), Simon Prosser

bbPress is forum software, made the WordPress way.

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

Your WordPress, Streamlined.

By: automattic, alternatekev, Andy Skelton, Andy Peatling, Andrew Ozz, and others

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

By: Donncha O Caoimh, automattic