NeL is a toolkit for the development of massively online universes. It provides the base technologies and a set of development methodologies for the development of both client and server code.
It contains for instance a 3d engine, a network engine, a sound engine.
NeL is working on Windows, Linux and Mac under GNU General Public License.
The Ryzom MMORPG is based on NeL framework and is the best example of what you can do with NeL.