A free (libre), open-source (GNU GPL), content management platform designed for group collaboration. Open Atrium is based on Drupal (running on Apache, with a MySQL database).
This document will walk you through the installation of what is known as a "LAMP" system: Linux, Apache, MySQL and PHP. Depending on who you talk to, the P also stands for Perl or Python, but in general, it is assumed to be PHP. I run CentOS on my servers