Birth control pill is a good one, but lack of access and various social and cultural disincentives to use it have resulted in a still growing global population. (Birth control in general, including the condom, is a better thing to look at.) It hasn't made any real dent in the birth rate on its own- those few industrialized nations that have slowed their population growth down merely use birth control as one mechanism among many to control growth. The actual causes seem to be increasing standards of living and various (again) cultural and social causes.