eCalc is a free online calculator that supports many advanced features including unit conversion, equation solving, and even complex-number math. The calculator is designed to work directly in your browser and requires no special plug-ins.
Input Mode (Algebraic vs. RPN)
The online calculator operates in either algebraic input (default mode) or RPN input. The calculator mode is set by clicking on the "ALG/RPN" symbol in the status bar or by changing the mode inside the menu dialog box.
There is a Windows desktop version of basic calculator. It’s completely free and it runs on your computer without requiring an internet connection. It has support for 6 languages.
Download Free Calculator (Windows Installer)
A Portable version of eCalc basic calculator is also there which allows you to run the calculator from a flash thumb drive, or right from your desktop without requiring any installation.
Download Portable Calculator (Standalone exe version)
Other Alternative – http://web2.0calc.com/