The Ileor MMORPG is a small MMORPG. The game is aimed at learning about the architecture of MMORPGs by building a simple one to play around with. With time, it might even be playable.
Ryzom Core is a toolkit for the development of massively online universes. It was used to make the commercial MMORPG Ryzom. It provides the base technologies and a set of development methodologies for the development of both client and server code.