Encalc is a free online scientific calculator that I initially tried more than four years ago. I revisited it this evening and discovered that some more features have been added to it. The two biggest enhancements to note are the addition of a graphing function and the addition of embed codes that you can use to put the calculator on your website. Try Encalc as embedded below.
Applications for Education
Scientific calculators don't come cheap. 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.