Good luck -- I, personally, don't know of any airlines that do this anymore. That's not to say they don't! I just don't know of any.
I've done it a number of times. Only once internationally. The procedure for me was: show up at the airport, tell them where you want to go and when. If they had room, they'd throw me on at the last boarding call. At the time it was $35 for my domestic trips. I don't remember how much my international one was -- I flew from Florida (the one with the horseshoe shaped airport) to Gatwick -- but that was Laker.
My domestic ones flew me everywhere -- for that price... why wouldn't you? The $35 was for each leg of the journey. So, for instance, if I wanted to go Cleveland to San Fran -- I'd have to pay $35 to get me to Chicago and another $35 to get me to San Fran -- since there's no direct flight there.
Hope that helps!!
-Lisa-
