Google Earth Enterprise (GEE), the server-based and offline version of Google Earth, has been released today on GitHub as open source software.
Earth Enterprise is a geospatial application which provides the ability to build and host custom 3D globes and 2D maps. Earth Enterprise does not provide a private version of Google imagery that's currently available in Google Maps or Earth.
The application suite has three core components - Fusion (which imports and fuses imagery, vector and terrain source data in to a single flyable 3D globe or 2D map), Server (Apache or Tornado-based server which hosts the globes built by Fusion), and Client (the Google Earth Enterprise Client (EC) and Google Maps Javascript API V3 used to view 3D globes and 2D maps).
GEE has long been the preferred choice among enterprises for 3D visualization and private mapping, but going open source means that the offerings' millions of current users will likely be joined by millions more in relatively short order.