Securing extra room in coach with a second seat.

Not too long ago, if you were traveling on coach and wanted the seat next to you for extra room, the only ways you could secure them was if you were a passenger with a disability or was traveling with a oversized musical instrument that would not fit in the overhead bins, or your own size landed you in the category called "passenger of size."
Now, many airlines are giving travelers the option to buy a second seat at the same price as the first one. Generally, a "reserved seat " document or an extra boarding pass will be issued for the traveler to show to potential suitors for that seat.
Some of the airlines that allow that practice are: Delta, American, US Airways, Continental, Airlines, JetBlue and AirTran, according to the Wall Street Journal. As you may have noticed, one of the major U.S. carriers that does not offer that option is Southwest Airlines. But that airline has been notoriusly in the news for making certain heavy person buy an extra seat under their "passenger of size" policy.
Since most airlines require that the extra seat reservation be done over the phone, you will likely be hit with a reservation fee.
But even with that extra fee, the savings compared to a first class seat ticket, can be very significant on most routes depending on what period you are flying.

For more, see WSJ.com