This isn't an enormous chain like The Olive Garden but they are in large cities, especially near water and by vacation destinations.

Phillip's Crabhouse

I just went to one in Baltimore, MD and they had a buffett for $25.00 a person. I do not usually go to buffetts but this consisted of crab legs, crabcakes, oysters, clams, mussels, shrimp, clam chowder, deviled crabs, clam strips, softshell crabs, hush puppies, and more and a full dessert bar.. it was amazing and so cheap for endless amazing seafood!