A few years ago I was getting into iOS audio development and I started blogging about CoreAudio. My website started getting some traffic so thought I'd put together a really comprehensive tutorial on how to play MIDI files on iOS. I started selling it on my site for around $19.99 and started making a couple of hundred dollars per month. Then one of the founders of http://www.binpress.com reached out to me and asked if I'd consider putting my component on their site. I decided to go for it and with the added exposure it started bringing in around $400 - $500 per month. Then I decided to spend my spare time making components to save other developers time on common tasks. I developed a piano plugin and then a chat component (http://www.binpress.com/app/chat-messaging-sdk-for-ios/1644). The chat component did really well so over time I built it up - currently it brings in about $2k per month in sales and loads of consulting work. Because of the chat component, I was approached by the Founders of Firebase because they wanted to shut down an online chat service they had called Envolve. They asked if I'd be prepared to make an alternative service and take on their customer base. I took on the project and developed a new chat called Chatcat (http://chatcat.io). Currently, I'm making about $4 - 5k per month in passive revenue from Binpress and the chat. On top of that I can easily make another $5k in freelance work. I'd definitely recommend this as a low risk path to generating a really stable passive revenue.