free airline tickets

Well not exactly...have you ever had a return portion of a ticket that you could not use? Maybe it expires befor you can use it or is going the wrong way.
Don't throw it away, here's how to keep it valid:

1. Book your return flight  on or near the expirey date on a day you judge the flight might be full, (friday night, sunday night etc).and turn up at the airport well before departure time with your passport, (no luggage).
2. Go to the ticket desk, (not check in desk) and thell them that you have changed your mind and you don't want to fly today and ask them to issue you a voucher for the flight.  If they agree then bingo and go home.
3. If not then check in and go to the departure gate and tell them again you don't want to fly today and ask for a voucher but tell them if they can't do it then you will fly today.   If the flight is full, don't take the first no, ask again every 30 mins or so.
4. If they refuse then hang around until near take off and then go home.

If the flght is empty it might not work, but remember they usually overbook flights and bump people so having you volunteering is saving them money and time.  This worked at Dorval with air canada on a flight to London, UK.