I launched ProducersForum.org 4 years ago. At the time there was a need for a “Yahoo Groups”-functionality that offered:
Sub-lists for working groups that would still be accessible/cross-searchable
It seems rather quaint now, but there was a need for this for community media groups in 2006. Built using Drupal, it is the only website I know that followed Zack Rosen’s Magic Groups recipe to turn Drupal into a mailing-list manager that could create an arbitrary number of email lists, allow users to post via email or via the website, and archive beautifully-threaded conversations on the website. Built with Drupal 4.7, I learned quite a bit in optimizing user workflows and smoothing Drupal’s poorly executed content creation interface.