From Ajay Menon:
Reor is a free calculator (licensed under GNU General Public License v3) which is crafted into perfection with a winsome interface. Reor has got Scientific, Statistical, Graphical, Financial, and many more tools/functions built-into it, does calculations with at most precision, and maintains an attractive interface. It holds Scientific Functions (Trigonometry, Logarithms, Factorial), Statistical Functions (Mean, Mode), Graphical Tools (Ploting graphical equations), Financial Tools (Loan, Interest), Mortgage, Unit Conversion, In-built Physical/Mathematical constants, Ability to store variables persistantly, Manipulate values as fractions, Full-fledged Result History, Function-to-Function assistance (Butler) built-in, Updater, Limit decimals, Digit grouping, Minimize to System tray and Linq. Reor, unlike other calculators, looks very pleasing. Reor is meant to look nice and simple. Instead of putting forth all the function and bloating the user all at once, it lets you choose the tools you want, so that you can maintain a clean, and easy workflow. Reor (the main window) always stays in top of other windows, enabling you to multitask. Reor minimizes to the notification area, and also has a tool called "Linq", in case you want to manipulate Reor even when it is minimized. All in all, it is the only calculator you will ever need.