WebGL Earth is prepared to be used on third-party websites and in projects developed by other people. The developers have two options how to use the source code of the WebGL Earth project for custom web applications, the API
and the codebase
With a few lines of code anybody can use the 3D planet from WebGL Earth project inside of his own web.
The sources contain also an example of a standalone application - in the subtree "src-app". You know the application already from the live demo website at http://www.webglearth.com/. Feel free to customize it, modify or use the code for inspiration for your own online projects.
The steps you should follow to setup WebGL Earth development environment are described on Github repository: