Writing a book is an incredibly challenging task, and one that can take years to complete. Once the book is written and published, it doesn’t take any effort to maintain – unless you revised it down the road due to a change in your field. The risk is that no one downloads or buys your book, but there is considerable upside if you manage to find a following. There are a number of places to publish books online these days like Amazon.
If you are a photographer looking to diversify your income stream, putting together styled stock photo packages can be lucrative. For example, a package of 15 wedding-themed stock photos for $10. You can then market this to any bloggers or businesses who are in the wedding business for their use (photos of different engagement rings styles are super popular). Through this method, it’s possible to make a continuous stream of income off of photos you’ve taken once (similar to a licensing deal).
Crowdfunding, the process of pooling dollars from a large population of people to fund a larger project, has significantly increased in volume over the past several years. According to Statista, $34.4 billion was raised in the United States through crowdfunding in 2015. In 2012, this number was $2.7 billion. That number continues to trend upward as new crowdfunding options become available.

​I’ve been into home décor lately and I had to turn to Etsy to find exactly what I wanted. I ended up purchasing digital files of the artwork I wanted printed out! The seller had made a bunch of wall art, digitized, and listed it on Etsy for instant download. There are other popular digital files on Etsy as well such as monthly planners. If you’re into graphic design this could be an amazing passive income idea for you.

Viral marketing or viral advertising is a business strategy that uses existing social networks to promote a product. Its name refers to how consumers spread information about a product with other people in their social networks, much in the same way that a virus spreads from one person to another.[1] It can be delivered by word of mouth or enhanced by the network effects of the Internet and mobile networks.[2]