Plasma is a 3D game engine developed by Cyan, Inc. and now released under an open-source GPLv3 licence. It was used by Cyan for projects including Uru: Ages Beyond Myst, Myst V: End of Ages, Myst Online: Uru Live, and Cosmic Osmo's Hex Isle. The open-source code is the same engine as used in Myst Online: Uru Live, and is currently used for Myst Online: Uru Live again.
The Plasma engine GitHub project includes the source code for the core game engine, some data files for the Myst Online: Uru Live again game, the Windows game client, a 3ds Max plugin for content creation, and several small utilities.
The server code has not been released under the GPL, and is not available. Unofficial open-source server options exist, including DIRTSAND.