BuddyPress.org

By the way, after casting about for a similar solution, I’m using a free widget from http://tellafriend.socialtwist.com (which will generate a plugin or sidebar widget code for you for WP), which has all the same functionality (plus more, including arbitrary email addresses as well as Gmail etc address book retrieval and Facebook and Twitter and the usual), throwing the code into an Advanced Text Widget (where you can add code as well as text — it’s a plugin from the WP repository), and putting the widget on the homepage. Works great.

Yeah, I know, security issues, but.