There is a number of calculators out there, and you can find one for practically any purpose you may have. Your choice should depend on what you are using a calculator for, and what kind of calculations you plan on doing.
Baldor Motors and Drives
United States Environmental Protection Agency
Beckman Coulter, Inc.
Prokon Software Consultants
Southern California Edison Co.
Dresden University of Technology
Thunder Heart Performance Corp.
ARES Institute, Inc.
Chipkin Automation Systems