Procamburus sp.. These beautiful blue crayfish reach about 4" long in the body. Like all crayfish, you can try housing them with fast, surface-oriented fish, but be aware that crayfish may eat anything they can get their claws on! They make perfectly interesting display animals on their own, though, and can very quickly be taught to accept food from their owner's hands/tongs.
This is a juvenile around 2" long in the body (with the tail curled).