A few years ago I turned some designs I had made for monthly art challenges and other personal projects into stickers. I can't say I've sold that many, but I love flipping through them, and once in a while I'll get a sale or two through Etsy or a local zine fair.
I also adapted the animal designs into a line of fabric designs on Spoonflower.
I'm participating in the same art challenge that resulted in the winky witch and skeleton keys design and a few of the designs I've made already feel like potential new stickers. Maybe I'll even be brave enough to try my hand at a pin this time around!