Autodesk Project Vasari is an easy-to-use, standalone design tool with analysis capabilities available via the cloud, built on the same technology as Revit, aimed at architectural designers and building energy analysts. It is focused on conceptual building design using both geometric and parametric modelling and supports performance-based design via integrated energy modelling and analysis features. It is compatible with Revit in that files created with it can be opened using Revit 2011, and vice versa. There are some limitations in that, as a conceptual modeller, Project Vasari does not support certain features, e.g., detailed model elements such as walls, windows, doors, etc. The thinking is that you would start with Project Vasari and then continue with Revit to make more detailed models, not the other way around.
This new technology preview is now available as a free download and trial on Autodesk Labs.
If you are like me, the short text introduction did not yet make it completely clear what Vasari is, but the sixty second teaser video is guaranteed to instantly change that.
Here is what some others have already said about it, so I will add nothing further: