Helpful seat maps and reviews
SeatGuru shows you a flight's seat map with reviews for specific seats and general indications of which ones are good or bad. It's a quick and easy way to help you find the best seat (or to at least avoid a bad one).
Get the exact seat you want
ExpertFlyer is like SeatGuru but on steroids: it overlays SeatGuru's ratings on top of the real-time airline seat data so you can see whether the good seats for a specific flight are still available. You can even set alerts for on your phone for when these seats become available.
For basic alerts it's free, but for advanced features it does cost money. For frequent travelers, we can see it being worth it.