SeatGuru is a site & app that lets you find the best seats on your flights, based on things like leg room and proximity to lavatories.

You put in the airline and flight # and get back a seat map of the entire airplane that runs that route. Seats are then color-coded with notes and sometimes photos/reviews from users, indicating which seats have the most leg room, are quieter/noisier, are accessible for families, etc.

While available as an app, the website is likely sufficient if you're planning all of your flights and seats prior to departure.

http://www.seatguru.com/