USAir charge $50 to give you a confirmed stand-by ticket (oxymoron but that's what its called). They basically check to see if there's enough seats, and if there is its a $50 change fee to the new flight.

I often schedule the latest flight of the day cause I am never sure if I will get out in time, something always comes up at the last second. In the past I used to be able to show up and say "hey I'm here... what's the next flight".