Tuxemon is a completely free, open source turn-based monster fighting RPG. Tuxemon has been completely written from scratch and contains all original code. In the spirit of other open source clones like SuperTux and SuperTuxKart, Tuxemon aims to create an open source game with its own unique style that sets it apart from other monster fighting RPGs.
Right now only a limit amount of assets exist for Tuxemon. If you're interested in helping create assets for Tuxemon, take a look at the art contribution forum for more details.
Note that Tuxemon is currently alpha-level software. Almost all of the game's content is still yet to be created, but much of the backend systems such as the event engine, combat system, dialog, and map systems have been implemented in some way. The current release has a two maps that you can walk around in that demonstrates some of the functions that have been implemented so far.