BuddyPress is a wordpress plugin that adds a social networking layer to an installation of WordPress MU. Many people like the idea of this plugin, but are unable to experiment with it because it runs only in single installations of WordPress rather than the multi-site WordPress MU.
To those people, good news! A little over a week ago Andy Peatling, founder and lead developer of BuddyPress, announced the release of BuddyPress 1.2, which can now be used on single installations of WordPress.
To keep your current theme with BuddyPress you can add The BuddyPress Template Pack directly from your WordPress admin, and it will walk you through the theme additions step by step. Now you can use BuddyPress with your single site installation of WordPress, and you can also keep your existing theme.