I've been working on Carlo, a tool that lets you do uncertainty and sensitivity analysis with Google Sheets spreadsheets.

Please note Carlo is an (expensive) commercial product. The pricing is aimed at professionals making important decisions. 

There's a lot more detail at the link, but in brief, some key features of Carlo are:

Works with your existing Google Sheets calculations

Variance-based sensitivity analysis

Our sensitivity analysis offers a true metric of variable importance: it can tell you what fraction of the output variance is due to each of the inputs and their interactions.

Flexible input

Inputs can be given using novel, convenient probability distributions that flexibly match your beliefs.