The Hecatonchires (Cottus, Briareos, Gyes) in The War of the Titans
Recap: The Hecatonchires are the sons of Uranus and Gaia. They are said to have fifty heads and one hundred arms each, and Uranus imprisoned them in the earth alongside their brothers, the Cyclopes, because he feared their power. Zeus frees them from prison during the war, and in return they join the Olympians during the struggle.
As far as we can tell the sole purpose of these bad boys is to help the Olympians whomp on the Titans. Briareos does show up in random places throughout literary history, but the three giants and their three hundred arms have no other role in Greek mythology that we can find. In fairness, beating the heck out of the Titans is an impressive accomplishment, and probably qualifies the Hecatonchires for retirement.