Microsoft Research released it's first version of the WorldWide Telescope in 2007, but its roots go back to 2002, when Jim Gray from MSR wanted to apply the lessons he had learned from creating the TerraServer earth mapping project to space. When it launched, it famously made Robert Scoble, who was working at Microsoft at the time, cry (giving it the dubious distinction of being the first of many software products to make Scoble cry over the years). In 2008, Gray would tragically be lost at sea in an apparent sailboat accident, and an at the time unprecedented search via satellite images was conducted by Gray's many friends and colleagues. The project carried on, however, and Microsoft Research has been maintaining the WorldWide Telescope ever since.
In a blog post, MSR announced that it would be open-sourcing WorldWide Telescope "in 2015", and gave this description of the project:
The WWT software was designed with rich interactivity in mind. Guided tours which can be created within the program, offer scripted paths through the 3D environment, allowing media-rich interactive stories to be told, about anything from star formation to the discovery of the large scale structure of the Universe. On the web, WWT is used as both as a standalone program and as an API, in teaching and in research—where it offers unparalleled options for sharing and contextualizing data sets, on the “2D” multispectral sky and/or within the “3D” Universe.
Now, however, MSR is beginning the transition from research project to open source with the creation of the OpenWWT Consortium, whose members "are institutions who will guide WWT’s transition from Microsoft Research to a new host organization". You can learn more in the links below.