WordPress is the world's best blogging platform.
See also
Anti Spam Blog
Automatically identify spam blogs
10 useful BuddyPress extensions
BuddyPress multi-lingual
Facebook Connect
WP-FB-AutoConnect
BP-FBConnect
Recommended by wpmudev
BuddyPress themes
WordPress MU (and BuddyPress) themes
WordPress themes
All themes are customizable, eg BuddyPress Social Theme
Forums
Request new themes, plugins and WPMU DEV features... we're always developing new things based on our members feedback!
Themes
Extensions
Manuals
Support
FAQ
BuddyPress themes
BuddyPress compatible plugins
BuddyPress manual
SEO
Supporter framework, powers Edublogs upgrades, works like WordPress.com
Supports BuddyPress
Affiliate program
Pay to blog (users must pay before blogging)
Auto blog (automatically posts items from RSS feeds)
Ad Sharing (allows you to split advertising revenue with your users)
We've developed plugins an widgets for global tags, blog directories, members directories, sitewide search, latest posts & comments and a whole lot more, and you can find them aggregated here for your convenience
Alongside that we've developed alternative functionality for sites that would prefer not to use BuddyPress, including:
A fully functional 'friends' plugin
A powerful communities framework, with wikis
A complete messaging component
A custom forums solution
But, of course, you need to have active blogs and users to fully realize the value of these features, and we've made it that much easier for you and your users to create and manage dozens, hundreds or even thousands of blogs and users... within a matter of minutes.
You can find our blog and user creation and management tools here:
Cache
Serve static .html cached pages
Cookies for Comments
Helps fight comment spam
YouTube
Allows <object> <embed> <iframe> tags (otherwise WordPress strips them)
WP-Hashcash
Helps fight comment spam
Google Analyticator
The best Google Analytics plugin supporting Google's new AJAX Analytics API
Webmaster Tool Verification
Supports Google, Yahoo and Bing webmaster tools
SEO
All in One SEO Pack
Google XML Sitemaps
The best sitemap plugin supporting Google, Yahoo, Bing and Ask search engines