Encalc is a free online scientific calculator. I'm not much of a mathematician so I'll share with you how Encalc describes itself on its homepage. "Encalc is an online scientific calculator. Its strength lies in its ability handle units and dimensional analysis, to define variables and its large database of constants. Parenthesis and scientific formulas are also supported."
One of the features that I really like about Encalc are the explanations of how different variables and constants function within an equation or formula. Click on the image below for an example of the explanations offered for this sample problem.
Applications for Education
Good scientific calculators are expensive. Some schools have the students buy them other schools provide them and sign them out just like a textbook. Either way someone is absorbing a large cost. Fortunately for schools and students facing budget crunches, there are web tools like Encalc available for free.