ProjectPier 0.8.5 and above have been released under the Gnu Affero General Public License is a free software license published by the Free Software Foundation. The GNU AGPL is similar to the GNU General Public License, except that it has an additional section to cover use over a computer network. It closes what is commonly known as the Application service provider loophole of the GNU General Public License. The additional section requires that the complete source code be made available to any network user of the AGPLed work, typically a web application.
Please see the GNU website or the attached version for more specifics.