You are permitted to fly standby for free on the immediate prior departure (even if it is a connecting routing that arrives after your originally scheduled flight). For any other flight option the same calendar day it is a $40 fee and subject to space being available.

More information can be found in these two recent threads, among others:
http://www.flyertalk.com/forum/jetbl...er-flight.html
http://www.flyertalk.com/forum/jetbl...y-standby.html