If you are a student, the International Student Identity Card or ISIC is very handy and good for many discounts of all kinds including flights and insurance. http://www.ciee.org/idcards/index.htm Cost: about $20.
If you are under 26 years old the Federation of International Youth Travel Organizations offers the GO25 Card with similar benefits even if you are not a student. http://www.go25.org Cost: about $20.
If you are a teacher, the International Teacher Identity Card offers discounts as well but not quite as deep. Cost: about $20. w.istc.org