There are several (commercial) organisations looking into alternative means to launch space-bound rockets. One commonly proposed method is to use a large airplane as the launch platform. This method should, theoretically, reduce the cost of sending rockets into space.
Weather balloons can reach an altitude of 20 KM, or more. Could a balloon be used as a launch platform for space-bound rockets? I imagine skipping the first 20km of flight could significantly defray the cost of a launch.