eCalc Basic Calculator
In Windows 7, the new Calculator was built from the ground up. The seriously spiffy new number-cruncher includes Programmer, Statistic, and Scientific modes, and comes with handy templates for figuring stuff like mortgage payments and gas mileage. If you're using Vista, XP, or even an earlier OS, you can get most of the number-crunching you need, free of charge, from eCalc.
Note: The vendor also offers a Scientific Calculator, which is a paid download.