For me it's really simple, I call my parents Mom & Dad and will refer to them as M&D with my sister. Sometimes I'll call my Mom 'Mother', but she doesn't enjoy that too much (haha).
What do you call your parents? Any special nicknames? Do you ever call them by their first name?
(Top photo from my vacation to Florida in 2012 & bottom from my holiday campaign with Place d'Orleans back in December 2012)