Before I turned to writing books, I spent some time trying to earn passive/residual income from other writing activities. While the terms are often used interchangeably (I do it myself), there is a slight difference between passive income and residual income.
In my case, I knew of several online sites that offered revenue-sharing for writers willing post content on said sites. One of those sites was Squidoo.com, and I eventually posted close to 40 articles there. I never earned a great deal of money there (I think the most I ever got was something like $40 one month), but it was fun and there was the potential for my earnings to increase down the road. Plus, my work was going to be out there earning something forever. But it turns out that "forever" wasn't as long as I thought it was.