Whatifster is an Excel add-in currently offered through Microsoft AppSource. As such, it has passed comprehensive vetting by Microsoft. In the near future, Whatifster will also become available as a web-app, allowing users to upload Excel-based models, together with the newly added analytic tools, to a proprietary cloud in order to view, share and operate them online.
The add-in is supported by Excel 16+, Excel 16 for Mac, Excel for iPad, and Excel online.
At the initial stage, Whatifster’s library of widgets contains only sliders to be linked to variable input cells. More widgets are on their way. Whatifster’s toolkit will expand to include additional input-related widgets, such as interactive multi-component bars, dials, and toggles. On the output side, Whatifster will offer uniquely designed charts and graphs not otherwise found in Excel.  
In parallel, ongoing development is focused on special-purpose tools that will add superior analytical capabilities to Excel:
Visual Goal Seek – “grab” and move an output chart to find the new values for respective inputs
Automated Tornado Chart – rank the inputs by their relative impact on a selected output
Multi-dimensional Scenario Analysis – create as many input/output combinations as you need and organize them visually