PlaneShift is an in-development 3D fantasy MMORPG by Luca Pancallo and Atomic Blue team. The action takes place in a huge kingdom set many levels underground in a deep cavern, with several races, mounts, many quests, and lots of terrain to explore (the cavern is divided into levels).
PlaneShift will be entirely free, without any premium features, and is open-source. Anyone can develop parts of the game themselves or even volunteer to contribute to overall development. A working tech demo version is playable via download from the official site and already has several hundred players.