This may not be such a hidden secret, but I came across this site. This shows the map of your aircraft, and tips (ie: misaligned window, etc.)
Please be aware that the airlines frequently change the seat assignment on the day of the flight no matter how far in advance you have made the seat reservations. This can be a real problem if you are traveling with a person(s) with different last names as they have no idea about who is traveling with whom!
This is one more reason to check in for your flight really early. I see people saying 2 hours is sufficient. I would never arrive less than 3 hours in advance.
The closer to the time of departure, the longer the lines and if you need a change of seat, you will have a slim chance of getting it.
Besides it is just less stressful to check in early. You are then free to get a bite to eat or whatever.
Great site. I enjoyed browsing through it. One thing it doesn't seem to show is a window seat with no window - just the fusilage bulkhead. When on planes, I take notes on good seats, bad seats and especially window seats where there is no window. I use this list when selecting seats on flights.
Great Site! Hint....review your air itin more than once before departure as the air carriers can change "equipment" which could change your seating.
I love that site. I never make a reservation without checking it.
The other website similar to seatguru is seatexpert.com. Some people have a preference - I will normally run a check with both.
Unlike one of the previous posters, I haven't had much problems with US based airlines changing my seat assignment. Now if there is an aircraft swap, there isn't much that can be done.