I recycle old and rejected works and sell them as vector graphics and icons at microstock sites, Graphic River and Creative Market. When I have more time I create some assets from scratch too.

Surprisingly the sales are pretty consistent. Because of the difference in licensing in microstock, a simple vector shape that I created for less than 10 minutes was purchased for almost $30. It's fun.

Other than that I sell one HTML template that has been making a few hundred dollars. It's not much but when you consider the small amount of time needed to create them, and you can sell them over and over again, the ROI is pretty good.

One of the tricks with passive income is you need to create multiple streams. Combined, they can serve as a great cushion during dry spells.

I'm in the process of writing a book teaching small businesses how to buy digital tools (websites, apps, etc) though I'm not really doing it for the money. Its more to get my message out there.

In the past I've sold things on the various creative networks (creative market, iStockPhoto, Envato) but I never end up putting as much effort into them as I should.

Over the years I've done a lot of things to generate passive income but I've realized that they never really go very far. The problem is that I'm not willing to put in the effort to make them viable. However I'll dump loads of time into things that are personally interesting. So try instead of looking for passive income sources, find a way to monetize that stuff you're going to do anyway Then it's a win win and you'll be personally invested enough to keep it going.

I teach bookbinding on Skillshare. Over the past two years I've created four classes. They do alright, but not amazing. After all, bookbinding isn't really something most people are interested in. But I've made a few thousand $ from the classes. The classes have about 3k students with a fair amount of participation. I kind of have the bookbinding market cornered, but the tough part is that bookbinding has little to do with my day-to-day life and profession in design. So I wish there was more overlap. Still considering whether I want to continue making more bookbinding classes or move to something else.

My classes: https://www.skillshare.com/calebsylvest/teaching

Besides that I'm always trying new things (that typically don't work), but the latest is a job board/networking thing specifically in Dallas. Dallas is a hot city (no pun intended) for technology companies, and I like to help people develop their careers and find good jobs. So been working with a friend of mine to connect talented people with great agencies. We really want to leverage our personal networks and connections to make the fit, because that way typically works better than randomly applying to jobs or hiring random people. The monetization side is still in flux, but hope to figure that part out soon.

DallasCollab: http://www.dallascollab.com/

I started http://uxtree.io a couple of months ago, it is a site with design courses. I have two free courses, and one paid course. I've made several thousand dollars off of the paid course in the past few months.

Shameless plug: If anyone wants to get their feet wet teaching a course on the site and earning passive income, email or tweet me! I am looking for instructors.

Investing

As a UX Designer I found that there's "job security" in analytics, landing pages, and A/B experiments to increase conversion ratios. Therefore, past clients that paid for my freelance / independent contractor ux/ui designs pay me a $250-500 retainer each month for analysis, a/b experiments, and conversion optimization.

My own eBook is in the works, http://bureaucraticux.com .

Something I learned is that it's never too early to "generate hype." You're leaps and bounds ahead of the competition especially if you conduct your own user testing.

I've also started a udemy course but have yet to publish it. I'll be charging $199 per student for the course. https://www.udemy.com/user/nate-chen/

I also know that https://www.redbubble.com is one way to sell your own t shirt designs online.

I am a marketer by trade and I run a bunch of small side projects that are monetized through ads : http://saijogeorge.github.io/projects.html ( please don't judge them too harshly, I am not a dev / designer and that site is a work in progress)

I have started teaching a free session through General Assembly and organize the Local Meetup for SEO in my city. You get to meet a lot of people in the industry and can network a lot, a really good career move IMHO.

I've been trying this out and seeing small but steady growth with no effort: http://www.protravelplus.info/bradenhamm

I've started an stopped creating Axure widget libraries (it's the prototyping tool we use at work and is a fairly underserved market), but I'm just not passionate enough about it.

I'm also working on a consumer facing product that will earn me recurring revenue, but I'm preparing for it to take some time before it's sustainable.