American Airlines and others now charge fee for flying standby – $50

Flying standby is a nice way to get home a bit early from that weeklong business trip. You get to the airport and hop on the next flight heading in the right direction. Since February American Airlines has been charging $50 to travelers who want to take advantage of that service. Frequent flyers, business class and first class are exempt of course.
http://www.aa.com/i18n/agency/Travel_Experience/same_day.jsp