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.
CASIO COMPUTER CO.,LTD.
Simplified Office Software Systems
Control Engineering Sweden AB
Ken Wilcox Associates, Inc.
Ars Nova Software
WeBBusterZ Engineering Software
SJW Computer Resources
Ezy Loan Calculator
Head Document Tool Software, Inc.