In addition to blog posts, I write short reviews [1] for technical books that I've read and use Amazon Affiliate links (a single link to the book).

It works out to between $20-$50 a year for my blog [2]. Nothing major but it covers a couple "free" Kindle books or a nice steak dinner :)

[1]: http://www.mdswanson.com/writeup/2013/11/13/how-to-fail-at-a...

[2]: https://gist.github.com/swanson/4711006