We want multiple lists - organizations, speakers, events, features that could be plotted on a Green Map etc - to all be posted online where others can edit and update them. Ideally it would have a CardScan or Plaxo-like service that tests for bad addresses and sends out update requests, preferably on demand rather than on a set schedule.

Take a look at CiviCRM to store the CRM data. CiviCRM already has geocoding APIs, so you could test addresses for USPS standards on save or you could create batch tool that would test addresses in the CiviCRM.

If you use Drupal or CiviCRM's profile functionality, it should be pretty straight forward to add a module that would prompt a user with a one-time login (so they don't have to remember their password) to update their contact information via email. It worl make more sense to allow you to choose a CiviCRM group to update contact information that to allow you to select Drupal users... that way you could update CRM contacts that are not necessarily yet Drupal users.

Drupal GMap module would be good here. CiviCRM Profiles allow users to edit their own information. Drupal modules views/CCK would work well here. There is a module CiviNode that allows you to expose data from the CiviCRM database as a node in drupal (i.e. your multiple lists)

This is a pretty cool and potentially complex project but very much in need.