Cornell is, in my opinion, the leading bird authority in the country. The Merlin app was created with the input of many, many thousands of people, nationwide, who participate in Cornell's "e-Bird" citizen/science project, as well as descriptors provided by the millions of visitors to their "AllAboutBirds.org" web site.
Another thing that sets it apart is its simplicity. It doesn't plunge the user into a mysterious morass of technical terms. It asks you for simple responses about the bird in question. First, the part of the country, and the date, where you saw it. Then the relative size of the mystery bird; sparrow-like or goose-like and so on. Then the dominant color. Then the location; on the ground, swimming or whatever.
Then, using your input, Merlin lists a few possible birds that it could be. It also gives you pictures of each possibility and the sounds each one makes (I really like this part).
It's incredibly accurate (for instance, it will exclude a bird from New England or one seen only in the summer). Also, it gives you range maps of the birds. And it can go into greater detail on the bird's behavior if you want. For instance you can find out that the actual Merlin is a small species of Falcon, occasionally seen in Texas and areas mainly to the west.
Right now, you can get Merlin at no cost - free! At some point in the future, however, it may have a price tag, for new buyers.