- I have a couple open source projects, and hosted "webmaster tools" type projects, with one-page websites like http://www.daterangepicker.com that get ~10K visits/day, and run one AdSense ad on each page, which brings in a couple hundred dollars most months.
- I run a web store where I resell another company's products (with permission) targeted at a different customer group than their own website. I advertise it on Google AdWords and Bing Ads on just a handful of keywords. Fulfillment is automated through the other company, so I haven't touched the store except to update the design every few years, and it brings in $500-$1000/month in profit. Sorry, don't want to link to this one, too easy to copy.
- Dividends from stocks and high-dividend-yield index funds are totally passive. But first you have to earn and save the money to invest in them. I also have some money in REITs, investing in real estate without having to buy and manage the real estate.
- I used to run some referral ads on my sites for a merchant account provider I used 5-10 years ago. A couple businesses signed up through those links and still use that company for their credit card processing. The commission agreement was a lifetime residual based on their monthly processing volume... I'm still getting $100-200 transferred into my bank every month from that company even though I haven't been referring anyone to them for years.