A stock portfolio that I've built through consulting profits is my main passive income source. Anything else is not really "passive." This probably isn't the answer you're looking for, but in my view the average developer is much better off simply selling their time at the maximum rate and funneling the profits into a well-managed portfolio. The median outcome for this is much better than the story on most passive-in…
Some of us would be willing to have a lower standard of living (possibly significantly lower) if a passive income could provide that.