These online graphing calculators have been popping up for awhile, but I am seeing e-Tutor's version get consistently high ratings by teachers in ed. publications. No registration or sign-in, extremely simple to use.
Enter functions of x using a standard mathematical format. You can enter more than one function by separating functions with a semicolon.
I entered sin(x) and got this result:
You can play around with it by changing exponents, constants, etc. Really cool feature - roll (don't click) the mouse over the line and see the x and y coordinates.