Showcalc is a Windows calculator with a scrolling tape so you can check your work. It has a huge range of functions including scientific, tax and a full featured unit conversion facility.
The scrolling tape display can be customised to your needs. You can change the tape's fonts and colors and numbers can be shown in different formats including your local currency. Also, for accountants, negative totals can be shown in red.
If you get a calculation wrong, you don't have to enter the whole thing again - you can go back through the calculation using the "Previous" and "Next" keys, enter the correct number and "replay" the calculation.
You can also hide the tape and/or scientific functions to give the calculator a slimer profile. Additionally, for people with vision difficulties, Showcalc has a "zoom" feature that lets you double the size of the calculator.
Note: This is a 45-day trial. Registration is $17.50.