Dansdeals.com has a great guide to the current standby policies of major carriers effective June 30th. Here is our summary version of it, or you can click the link. The Standby to which we are referring to is when one has a confirmed ticket and wishes to stand by for an earlier flight.
A few years ago, the airlines came up with a variant of standing by for earlier flights…confirmed standby for a fee. American, AirTran, Continental, Delta, Frontier, JetBlue, Midwest, Northwest, United, USAirways, and Virgin America offer this option for one fee or another. Southwest offers no standby. Changes for payment to the new fare only. Free standby is still offered by many characters, with the exception of Continental, Delta, for JetBlue except immediately before the one you are scheduled on(which is still free), Northwest(except for Skyteam Elites), and USAirways except if confirmed standby is unavailable.
If standby has to be for a fee, we think the JetBlue system is the most reasonable. Free for the flight right before yours, and available to be confirmed for a flat fee earlier in the day.