RedCrab is a versatile scientific calculator with a host of powerful features.
The program allows you to work in a very natural way, for instance, entering formulae in a full screen editor. You're able to enter complex equations with fractions, exponents, integrals, square roots and more. And you're not restricted to just one: the program will accept several equations on the same page, with the result from one being carried to the next.
If this sounds complicated, then it can be, depending on what you're doing. But RedCrab is generally very smart about how it interprets your expressions, and in most cases it'll figure out what you need first time. And when it does, you can then save and load complex formulae for easy reuse later.
You don't have to get involved in complex math every time you use the program, though. RedCrab also works well for more basic calculations. And it's portable, too, so if you need to do math on the move then there are few better options - just install it on a USB flash drive, then call up the program when you need it, on any convenient PC.