web2.0calc.com is a free online scientific calculator that I have been using for a while now and have shared with my students. I was surprised when I realized I had never blogged about it.
It is free and easy to use. It has all of the functions most students would need (outside of calculus) and shows the entire calculation in the top of the screen.
It can even be accessed by most mobile browsers so students with smartphones don't even need to buy a scientific calculator app for their phone.
There are also widgets available so you can embed it in your blog or website (which I did for my students here).
Great free resource.