Guesstimate

Guesstimate "A spreadsheet for things that aren't certain."

From the blog:

This is why I’ve made Guesstimate, a spreadsheet that’s as easy to use as existing spreadsheets, but works for uncertain values. For any cell you can enter confidence intervals (lower and upper bounds) that can represent full probability distributions. 5000 Monte Carlo simulations are performed to find the output interval for each equation, all in the browser.