Options Away is a site where you can book flights online and lock in the price for days or even weeks before committing. This is ideal if you find a great deal but still need time to confirm your travel plans or keep your options open.
There is a small fee for holding the ticket, depending on how long you are holding the ticket for. The service is like an insurance premium on cheap flights. If you don’t end up taking the flight there is no penalty. The service is currently available for US flights with more coming soon.