If you use Adobe Illustrator a lot this might be handy for you. You can do multiplication, division, addtion and subtraction within the app:
As the screenshot shows above, I have a 10mm box and I want to increase its width by 2. So in the ‘Width’ field I simply type in ‘10mm * 2’ and voila. Granted this is not that usefull for a simple operation like this however, if you find yourself often using decimals (e.g. the golden ratio) this is a time saver.