K2 is the popular powerful content component for Joomla! with CCK-like features developed by JoomlaWorks, makers of fine Joomla! extensions since 2006.
It provides an out-of-the box integrated solution featuring rich content forms for items (think of Joomla! articles with additional fields for article images, videos, image galleries and attachments), nested-level categories, tags, comments, a system to extend the item base form with additional fields (similar to CCK for those acquainted with Drupal), a powerful plugin API to extend item, category and user forms, ACL, frontend editing, sub-templates and a lot more!
Using K2, you can transform your Joomla! website to a news/magazine site with author blogs, product catalogs, work portfolio, knowledge base, download/document manager, directory listing, event listing and more, all this bundled under one package! Since K2 is extensible with additional fields to its base item form, you can easily create category-specific content types, e.g. article, blog post, product page, directory listing.
View some highlights of K2 in less than 4 mins: Check out the K2 movie