WordPress themes aren’t the only things for sale through the Envato Market. You can also sell stock photos through PhotoDune, stock footage through VideoHive, sound clips through AudioJungle, anything graphic related though GraphicRiver, web and app development resources through CodeCanyon, and 3D elements through 3dOcean. There are a huge amount of options here for anyone with the skills to create resources.
The appeal of these passive income sources is that you can diversify across many small investments, rather than in a handful of large ones. When you invest directly in real estate, you have to commit a lot of capital to individual projects. When you invest in these crowdfunded investments, you can spread your money across many uncorrelated real estate ventures so individual investments don't cause significant issues.
Although this article has been written in 2007, now in Feb 2014, I still find it full of great advice and tips. What I learned from this article is that when writing a review I should focus on how this product helped me achieve X instead of simply repeating and enforcing the features and benefits of the product that can also be found on the sales page.