1.) Gauss's Law/Ampere's Law - the AP writers love to give problems that ask to find the electric field or the magnetic field about an enclosed current or charge. Knowing how to use these equations will pick you up valueable points in the free response.
2.)Right Hand Rules- if you cannot use the right hand rules correctly, you might as well throw away a couple questions on the test and hurt your chances at a 5 on the AP test.
3.) Kinematics equations with angular translation- I can't even tell you how many times knowing these will get you out of a sticky situation in the mechanics section of the test.