I have a few add-ons that do more, look better, or that I use more but I would still call this calculator number one. I'm very bad at maths and have problems with short-term memory, so even just remembering numbers to copy into the Windows default calculator can be difficult and time-consuming. "5,91... Uh. *switch windows to check* 5,92... Uh... *switch windows to check again* 5,923 plus... Uh..." With the status-bar calculator, I no longer have to remember long strings of numbers or even switch windows: I can do it all right there on the same screen!
Whether it's to check if that grocery store bargain really is a bargain or something more complex, this calculator makes life so much easier.