The setting of Planeshift appears to be inspired, in part, by the Planescape setting of Dungeons & Dragons; particularly, the city of Yliakum in Planeshift bears similarities to the city of Sigil in Planescape.
While the engine (Crystal Space) of the game is free software under the GNU General Public License, the artwork and game rules source code is licensed under a custom license (the "PlaneShift License") that prohibits all modifications and redistributions.