Hundreds of headache remedies are out there (eg aspirin, paracetamol, ibuprofen).
A good one to use on the streets is an accupressure remedy. Firmly grasp the part of the hand between the thumb and forefinger and massage in small circles headaches should start to ease within a few seconds. This is a good one because a person can do it to themselves. The same remedy has also worked to help calm panic attacks and relieve general tension.