In reply to NorthenClimber:
I used one a while ago (2002 I think) with Continental to go Seattle. Rules on mine were that you had to book a return date when you purchased the ticket, but you could change it as often as you liked, for free, over the phone, up to 36 hours before you were scheduled to return. I think the return portion had to be used within 6 months of departure.
Initially I planned a 3week stay, during my third week I extended to 4, during 4th week extended to 5, during 5th to 6, during 6th to 7, eventually came home after 8 weeks having changed return date 5 times.
Never had problems with flights being fully booked, any Continental route from Seattle to London (any airport) was a free change option.