3) Expert Flyer
On Expert Flyer, you can find seats for sold out flights, which can be a savior in a pinch of layovers and missed connections. This site operates by allowing travelers to set alerts, such as specific seats or two seats together, for a specific flight.
Then, once the seat is found you are emailed with an alert so you can buy the seat or change your seat via your airline.
In addition to offering alerts for seating, Expert Flyer uses ratings taken from Seat Guru to help you choose the most optimal seat. Keep in mind that Expert Flyer is not free for all. While you can get the first alert for free, which is ideal for once a year vacationers looking to score a sweet seat on their annual flight, you will need to purchase additional alerts. These will set you back 99 cents each, or you can upgrade your Expert Flyer plan to a monthly account for $4.99 a month.
You will receive additional features with a paid Pro account including flight alerts, the mobile edition, saved queries and flexible searching.