groups
Twitter Groups! Home
Until Twitter does groups, this is a decent option.
Organic groups list manager | drupal.org
This module adds mailing list functionality, so one might be able to effectively replicate the good bits of Yahoo! Groups.
Organic groups | drupal.org
I’ve been thinking about drupal for a long time. This module/extension/whatever they call it might be just what is needed for a project I’m looking at.