1- Desmos
This is one of my favourite graphing calculators. It does not require any software installation and no special hardware. It works right from your web.
2- Encalc
Encalc is an online scientific calculator. Key features include the ability to handle units and dimensional analysis, to perform symbolic algebra and calculus, to define variables, and to lookup values from its large database of physical constants. Integration, differentiation, parentheses and scientific formulas are also supported.
3- Web 2.0 Calc

This is another awesome online calculator. It is web based , so you do not need to install anything to get it working.
4- SpeedCrunch
SpeedCrunch is a fast, high precision, and powerful desktop calculator. It is available for Windows, Mac, and Linux.
5- Microsoft Mathematics 4.0