Web2.0calc is a free online scientific calculator. While it won't replace the most expensive scientific calcualtors, it can do what your average high school student needs it to do. The best part is, you don't have to use it on the Web2.0calc site because they offer three widgets that you can use to embed the calculator into your own blog or website.
One of the problems the mathematics department in my school faces every year is "disappearing" scientific calculators. Web2.0calc could solve that problem by being placed into teachers' websites where students can use them anytime they like without having to sign-out a calculator for use outside of the classroom.