Take a look at Drupal6 www.drupal.org
Its in the debian repositories so installation was easy. I haven't done much with it yet other then setting up photo galleries, but there's a lot of modules/themes/documentation to suit a lot of needs.
With all due respect, this link you gave is to the main pages, one has to troll through all the info to get to where one wants to get to.
When I give a person a link to a specific web page I always navigate to the page that offers the download or information, which makes things a lot easer for the end user.
As of yet I cant find a download link for Debian Squeeze. There is one for Debian Lenny, will this work with Squeeze?