Easily post syntax-highlighted code to your site without having to modify the code at all. As seen on WordPress.com.
IntenseDebate comments enhance and encourage conversation on your blog. Build your reader community, increase your comments, & boost pageviews.
HyperDB is an advanced database class that supports partitioning, replication, failover, and federation.
You can have simple, concise stats with no additional load on your server by plugging into WordPress.com's stat system.
Akismet checks your comments against the Akismet web service to see if they look like spam or not.
Display tweets from a Twitter account in the sidebar of your blog. As seen on WordPress.com.
WP LaTeX creates PNG images from inline $\LaTeX$ code in your posts and comments.
Add a contact form to any post or page by inserting "[contact-form]" in the post. Emails will be sent to the post's author. As seen on WordPress.com.