The NumWorks graphing calculator

You only need it, if the instructor is too lazy to pick problems that can be solved with pencil and paper. It doesn't take much more effort. But it does take non-zero effort when designing the test.

And then of course, you could have a professor like the one I had in my first year in university. For some inscrutable reason, he thought it was a good idea to give 20% of the test score to anybody who successfully computed the value of a 4×4 determinant. I still don't follow his thinking on that.