I’m going to revise my answer; I wouldn’t call anyone, since I’m currently unemployed, as of Wednesday afternoon. “Market Conditions”, apparently.
I would thank my boss for all the opportunities, politely file a resignation, and work out my required notice under my contract.
Bummer. Sorry to hear that. 
I would ask for a significant raise and a new job description.
And if I got it, I would hire a housekeeper, chauffeur, and dog walker.
I don’t think I can post what I’d like to say in this forum, it not being the pit.
In reality of course, I’d probably just send him a text saying that’s it, I’m gone, as I’m on 0 hours and have no shifts agreed right now. I’d then get a new phone number so he couldn’t just call me 6 months later and try get me to come back just for the weekend…
For my other very part time job, I would just tell them I’d like to donate any future wages, as it’s a nice horse rescue charity, and they’re all lovely.
I work for a small charity which is consistently running at a loss. I don’t know if I’d want to tell my boss that I’m now rich, and potentially funding the organisation, but i’d want to make it sustainable, so:
Erm, so. This is going to seem random, but i’ve recently come into contact with a very rich person who’s supportive of this organisation’s cause. They want to donate a very large amount, or a smaller amount over a few years if that works better for you, to ensure that the charity can become more financially stable. It’s virtually no strings attached, but they would like to ensure that all staff salaries are in line with the cost of living (so if there have been no payrises in the past couple of years to keep in line with the cost of living they’d like all salaries to be adjusted accordingly). We’ll need to chat about how much money we need to get to a point of stability and how and when the charity wants to be paid. I’m the intermediary - the donor wants to remain anonymous.
Roughly a year later, when I’m ready for something new:
So, i’m thinking of retraining to become a counsellor/going back to university/etc. I’m therefore giving my notice.
…I retired a few years ago, but still work part-time.
Of course when my job is teaching chess, roleplaying and computer games, why would I give it up?
Same thing I told him about 4 hours before I retired a few years ago. I said " Don’t take this personnal but go fuck yourself"
I’ve always said that if I found myself in a situation like the OP describes, I’d just go away and do what I wanted and in a year or two or three people would say “Hey, you remember Bo? Whatever happened to that guy?”
I’d tell him thanks for not being anything like the previous guy
I’d say it was a pleasure working with you.
Self employed now… powerball grade cash, I have several ideas
The huge medieval themed bed/breakfast and wedding/event venue. Along with serving as the ultimate SCA event facility in the western US.
An atheist centric college fund/charity.
Couple mil to where I didnt care if we so much as broke even and could live fine with no salary from the biz., I would probably turn my shop into a boutique builder and have fun building the coolest gaming computers on the planet.