Content is interesting to me because the monetary value of an individual piece of content in most cases is basically zero, so the value ends up coming from the way you curate, organize, and present a specific type of content.
One site I created a while back curated photos of wedding bouquets and organized the photos by the types of flowers that were in each bouquet. Another that I am currently running for fun organizes a specific type of information related to the city I live in which is really only useful to people who live here.
I periodically blog about iOS development, compiled few posts (and added additional chapters not available in the blog) into an ebook and sells them.
Wouldn't say substantial money but I have earned $1k+ from selling a $29 ebook