Community Builder is an entire suite of Components and Modules that enable you to turn Joomla! into a community website with user pages and a high degree of user interactivity. There are a large number of Modules and Plugins available for Community Builder and integration of the Extension is widely supported by other common Extensions. This is a non-commercial Extension, but it does require registration to download.
The JomSocial Extension enables you to turn your Joomla! installation into a full blown social networking engine. The Extension is still relatively young, but the current release is feature rich and shows a great deal of promise. It is certainly your easiest, fastest route to a social networking website.
AddThis Social Bookmarking
Integrates the AddThis social bookmarking button to your site. This is a non-commercial Extension.
JReviews is a powerful Extension that lets you create a reviews and ratings website with Joomla! The system is customizable and can even be used as an alternative method for handling your Joomla! content items, with custom structure and fields. This is a commercial Extension.
plugin Googlemaps is a Joomla! Plugin that integrates Google Map functionality. Not only does it allow users to view Google Maps, but also includes support for MKL files and marker placement. Directions are integrated and can appear in popup or lightbox. This is a non-commercial Extension.
MyBlog provides greatly enhanced blogging functionality for Joomla! The default Joomla! system allows you to create blog-type layouts, buy MyBlog gives you true blogging functionality with a wide range of common features. Includes RSS feeds, Technorati pings, Trackbacks and support for multiple bloggers. Integrates with both Community Builder and JomSocial. This is a commercial Extension.
Tweetme is a simple Extension that adds a Tweet This button to your Articles and items. This is a non-commercial Extension.