There are a few tools out there to help you find the best airplane seat, such as SeatGuru, SeatExpert or SeatMaestro. ExpertFlyer also offers some tools, but only one I've seen so far can actually get a better airline seat for you. When I say get you a better seat on the plane, I mean actually book you in a better airplane seat. MySeatFinder.com is unique in that it will look at your self-selected airline seating preferences and compare those preferences to your assigned seat on your next flight. If a better seat is found, it will automatically move you. For example, you may be a middle seat in the back of the plane and you prefer to sit in an aisle or window seat, preferably more toward the front. If MSF finds you that seat, you're rebooked.
MySeatFinder is currently a free service. It's a seat finder only, there are no more tricks it does. But considering that where one sits is the second most important attribute of a person's flight (price of a ticket is #1), it's a pretty important job. Their tag line is: "finding, and getting, you a better airplane seat." Give it a try.