Open Source Tools
I frequently try out and contribute to a variety of open-source online community-type tools including Mambo, Tiki-wiki, Drupal (a commercial version of Drupal = Acquia ), and Moodle. I have several currently installed and use some of them for testing out comparative features.
Several guides on the Web provide discussions of the various tools available. Wikipedia provides one of my favorite general lists here.
This one is by technology platform: http://www.la-grange.net/cms
This one allows for feature-level comparison including non-open-source: http://www.cmsmatrix.org/
This one is more about popularity: http://en.wikipedia.org/wiki/List_of_content_management_systems
Here are some links to tools I have installed and used for experimentation or research: