Power BI Desktop – currently known in preview as the Power BI Designer – is a new visual data exploration and interactive reporting tool available for free direct from the vendor. It provides a free-form canvas for drag-and-drop exploration of your data and a library of interactive visualizations, while hoping to streamline report creation and publishing to the Power BI service.
Microsoft is opening its commercial grade visualization framework and visuals built on D3.js, enabling customers and partners to extend and build custom visuals. Along with the visualization framework they are also delivering a test suite and tooling. All of this is available today as an open source project on GitHub.