Not so much a actual words as descriptions:
Paternal grandfather: “Farfar” = Fatherfather
Maternal grandfather: “Morfar” = Motherfather
Paternal Grandmother: “Farmor”= Fathermother
Maternal Grandmother: “Mormor”= Mothermother
Alternatively, there is “Bestefar/ Bestemor” (Best Father/Best Mother)for either one. Or even “Bestepappa/Bestemamma” (Best Daddy/Best Mommy).
Slight hijack, but I think It’s nice too.
Why not throw in a Spanish “abuelo.”
And if you eventually run out of 'em, you can call him “Zebedee.” That’s was my grandpa’s actual name.
Icelandic: Afi’ -grandpa 'Om-ma (think “umma”) -grandma
My grandfather was of Italian descent, and we called him “Nonno”.
My nephew uses Grandad and Grandude to distinguish between the two. I think Grandad is a little jealous of Grandude 
It looks like I’m destined to be Papaya (or Papa) while my wife will be Banana (or Nana)
**Beepa ** was my grandfather.
Another vote for Pépère.
My uncle is just plain Pap to his grandkids. Everybody else I know is Grandpa.
My father is “Bobo” to my nieces and nephew. The etymology is a bit convoluted.
My daughter called my father “rah rah.”
I always liked “popsy,” especially after the Stephen King story.
These are excellent!
I just had the one grandfather, and he was Grandpa.
My father refused to be called grandpa and taught my oldest nephew to call him “Jaja” (Polish–roughly phonetic spelling; the actual correct spelling is something like “dzadza” or “djadja”), and that eventually got morphed into “Jaj”.
I called my paternal grandfather Granddad (we weren’t really close) and I call my maternal grandfather Papa. Not very creative, me. 
In these parts, it’s common to hear Grand+FirstName. Popularized by Caroline Kennedy Schlossberg’s children’s “Grand-Jackie”
My grandfather was Poppa. Ugh, I miss him
(I was raised by my paternal grandparents & Father. Grandmother was Mom (died when I was 18 (1991), Grandfather was Poppa (died almost 3 years ago) and my Father is Dad. I never met my maternal grandparents)
Grandad and Abuelito FirstName…
My wifes father was called dzai dzai . It is pronounced ja ja . We called him two jas.
Some are called teflon don.
My grandfather on my dad’s side was Poppop. Granddad on my mom’s side was just Granddad.
The Yiddish word for grandfather is zeyde/zayda/zeidi (I have no idea what the standard English spelling is).