Guesstimate is a new free/open source project from Ozzie Gooen: unlike normal spreadsheets, its cells accept confidence intervals, and it outputs models based on 5,000 Monte Carlo simulations.
The whole thing runs in your browser.
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.
At the end of this you don't just understand the 'best' and 'worst' scenarios, but you also get everything in between and outside. There's the mean, the median, and several of the percentiles. In the future I intend to add sensitivity analyses and the value of information calculations.
Introducing Guesstimate, a Spreadsheet for Things That Aren't Certain
Guesstimate [Github] [MIT License]