I run HTTP Toolkit ( https://httptoolkit.com ) which passed $2k a couple of years back. No longer a side project, as it's made enough money for me to work on it full time for a fair while now, but it certainly started that way, and it's still a one-man show (plus many wonderful open-source contributors). I suspect that'll be a common theme in answers here though: if you have a side project making $2k a month, in most…
how is this different from devtools? is the main selling point easy of use, or more extra features on top of devtools?
For the web dev case, for example, if you're debugging some interaction that means you can intercept your browser server traffic and your server upstream API traffic all in the same place, and see the full flow, and you can modify server responses or backend API responses in flight, to test out different edge cases.
There's a Chrome dev tools vs HTTP Toolkit comparison page here with a little more detail: https://httptoolkit.com/chrome-devtools-alternative/