Back when I was studying physics, we frequently had to do calculations with error propagation. I tried to implement something very similar in C++ and in Python, but never finished it. I also thought it would be neat if a spreadsheet program could understand uncertainties, and also units, so you could enter 1m +- 10cm and it would propagate the errors correctly. If you laid out the data with one column for the values and one for the errors, I had a couple of OpenOffice macros that would perform the calculations.
Another place where I think this would be neat would be in CAD. Imagine if you are trying to create a model of an existing workpiece or of a room, and your measurements don't exactly add up. It's really frustrating and you have to go back and measure again, and you usually end up idealizing the model and putting in rounder numbers to make it fit, but it is less true to reality. It would be cool if you could put in uncertainties for all lengths and angles, and it would run a solver to minimize the total error.