I call my husband’s parents by their first names. They are less than 15 years older than I am, so it makes sense.
He calls my mom “Omi” which is what all the grandchildren and in-laws call her. It is a German form of Grandmother, and since my sisters had their children, it is what most people in the family call her. Thank goodness, cause I can’t see Rick calling her Rosemarie.
I actually call my husband’s parents “Mr. Surname” and “Mrs. Surname”, as does at least one of my brothers-in-law (now that I think of it, I think both of them do). I followed the brothers’-in-law leads in what to call them. I also do this to some extent because they’re 40 years older than I am, and calling them by their first names would feel weird.
My husband sometimes calls my mom “Mom” or “Mom Lastname”; other times he’ll call her “Mrs. Surname”.
First names.
But my mother had a relationship with her mother in law (still does) something like Goo’s. I think she had kids so she could call her Grandma. I’ve never heard my mother address my grandmother as anything other than Grandma.
(Since I was born when my Grandmother was 39, this was particularly harsh - and perhaps one of the reasons they still don’t get along).
(BTW, Goo, knowing your perferred childless state, I’m not recommending this as an option. I mean, it would give you something to call her, but I don’t think its a really good reason to have kids).
I’ve been married for 7.5 years, and I still don’t really have what to call them. In person, I’d get their attention and just say what I want to say without addressing them. On the phone, I’d say, “Hi, it’s Mattis.” and then proceed.
They’ve never invited me to use their first names, and I just can’t get comfortable calling anyone other than my own parents “Mom” and “Dad.” It’s a fine relationship, but that one thing is a little odd.
I call my wife’s parents “Papa” and “Mother”.
Not sure why… just worked out that way.
My wife doesn’t call my Mom anything… to her face, anyway.
Hehehe, no worries, Dangerosa there’s plenty of other names I could come up with to call her, before I’d have to resort to ‘granny’ 
Actually she is getting better. I’m using this Christmas as a litmus test, so family relations may be harmonious, or you may see me in the pit 
It’s interesting to see what everyone calls their in-laws… Looks like I’m not the only one with in-law ‘issues’, but it’s also good to see a lot of people who don’t seem to have problems, too.
I call my mother-in-law “omo-nim” and my father-in-law “abo-nim.” I’m not sure what it means, but I think it’s something like “honorable mother” and “honorable father.” It’s what my wife told me to say. Whatever I say to them, they usually just laugh in response. I guess my Korean pronunciation is a little weird.
My wife has yet to call my parents anything. She’s only met them twice, and so far she usually just smiles and nods. Wonderful woman, but her Engish is not good.
My sisters are both married, so that gives me two brothers-in-law. One calls my parents by their first names, and the other calls my father by his first name and my mother “Granny.” My mother doesn’t seem to mind, but then she wouldn’t complain if she did.