Drupal

Open Source CMS

http://www.drupal.org/

http://drupal.org/features

lots of Virtual Community stuff: Weblog Engine, polls, Discussion Forum

written in PHP

Civic Space is a distribution of Drupal for online organizing.

http://www-128.ibm.com/developerworks/ibm/osource/index.html

Acquia, founded by Drupal founder, does commercial support http://ostatic.com/blog/acquia-supporting-the-drupal-cms-adds-200-new-customers

Press Flow is a semi-fork of Drupal providing better Scalability.

It seems that you can run on MsWindows, even with MsSqlServer. (Unlike Word Press, which will work with MsWindows but only wants MySQL for a db.)