A Texas Instruments graphing calculator. Basically allows one to get an A on any test in high school math, including calculus if you have even the slightest idea of what you are doing.
Best part is it can help you speed through anything you already know from factoring to limits (it even has an infinity button) to get to the good stuff. Considered unfair by many.
Better than TI-92 because it doesn't have a qwerty keyboard so its legal on SAT, Academic Decathlon, etc.