Opensocial

Below a list of modules that rely on Opensocial ranked by date of the last commit.

http://drupal.org/project/gigya (2009-Jul-31) *
. covers Facebook, MySpace ID, Twitter and any OpenID
. invite external friends to site
. share events, tweets, status, newsfeeds from site
. admin announce events, tweets, status, newsfeeds to social networks
. embed user profile
. embed social graph

http://drupal.org/project/fbconnect (2009-Jun-10, redundant if Gigya used)
. covers Facebook
. see registered friends
. invite external friends

http://drupal.org/project/friendconnect (2009-May-12, no official release yet, but definitely worth installing) *
. covers Google, Yahoo, AIM and any OpenID

* best choices if combined.