Hades' cabin (#13) was built at the end of The Last Olympian. The cabin houses demigod offspring of Hades. It was mainly designed by Nico di Angelo and constructed with help from summoned undead workers.
Hermes cabin (#11) is the cabin that houses the children of Hermes and newcomers to Camp Half-Blood who are unclaimed. Formerly, the children of minor gods lived there as well. New arrivals still do stay at the cabin until they are claimed.
Athena's cabin (#6) is the cabin that houses Athena's demigod children. All members are "brain children" (meaning that they are born from the thoughts of the goddess), which makes them the only demigods to be claimed at birth.