I'm not a grandparent yet, but DS calls one set Mema & Poppa, and the other Grandpa & Nana. (And we refer to my mom, who died before he was born, as Grandma Firstname.) He has known two great-grandmothers, one called Mema Firstname, and the other simply Gram.
My own grandmothers were Grandma Lastname and Grandma (cutsie form of last name). I didn't get to know my grandfathers, so my great-grandfather was just Grandpa.