Not a start up per se, but my Medium blog is fairly profitable, ~2k on a good month.
Mine is about, uhm, me being a disillusioned 30 year old developer hating how things are done now? Rants like this https://medium.com/commitlog/how-to-design-for-the-modern-we... tend to alright.
Throwing in an amazon link when its appropriate, or funny in the above case doesn't hurt either. Kinda got an on-going thing with always pushing K&R books.