Major modules for which a quick summary can't do justice

Organic groups, aka OG (http://drupal.org/project/og)

Enable users (or site builders) to create and manage groups (or semi-independent site sections). Groups may be private, public, or include a mixture of the two. List of (many, many) related modules: http://drupal.org/project/modules?filters=tid%3A90&solrsort=sis_project_release_usage%20desc

Panels (http://drupal.org/project/panels)

Requires Chaos tool suite.
"Allows a site administrator to create customized layouts for multiple uses" from within Drupal, using a drag and drop interface.

Panels Everywhere (http://drupal.org/project/panels_everywhere)

Requires Chaos tool suite.
An "advanced method to [replace] Drupal's … blocks system and instead use the … Panels Layout system to control how your pages look."