I run https://IndieHackers.com where I interview developers/entrepreneurs about how they make money from their apps and projects. There are tons of examples there that fit into the "passive income" category as you define it ("making something that makes you money while you sleep").

A couple of good examples from the site:

- park.io (from Mike Carson) makes $125,000/mo taking domain backorders; he essentually wrote tons of scripts that buy expiring domains on behalf of his customers for $100 and that allow customers to bid against each other

- Instapainting.com (from Chris Chen) makes $32,000/mo allow customers to commission artists to turn their photos into hand-made paintings; it took him years to get to this point, but it's almost all automated and he rarely has to step in

Indie Hackers itself is my passive income source, although right now it takes a lot of work on a day-to-day basis. I'm writing a blog post about that that I hope to put up on my timeline (https://IndieHackers.com/timeline) by tomorrow. I'm doing just over $1000/mo right now, mainly from sponsors.