Wow! What an awesome list! My favorite is the stock photography because I love photography. I have had some success there, particularly with one photo I make some decent income from. I think the key with stock photography is finding a shot that is high demand. Then, find a new unique way to frame that shot. This is the reason my St. Louis Arch photo is a top 10 on both ShutterStock and iStockPhoto. Thanks for the awesome ideas above!
Truebill is an app that helps you save money by identifying recurring subscriptions and other bills and helping you cut costs by negotiating better rates and fees. One of their partnerships is with Acradia Power, which has the potential to save you up to 30% on your electric bill. It searches for better power rates in areas where competition is allowed, and it locks in the better prices for you.
I just wanted to say how nice it is to see such a positive exchange between strangers on the Internet. Seriously, not only was this article (list) motivating and well-drafted, the tiny little community of readers truly were a pleasant crescendo I found to be the cause of an inward smile. Thank you, everyone, and good luck to you all with your passive income efforts!! 🙂
Camps, ski resorts, cruise ships, airlines, expeditions etc all may have need for part-time or full-time medical expertise.  Part of the compensation may be taxable income, or it may simply be opportunities.  For example, I work at a ski resort several days a month.  I take runs and a P.A. sees the patients.  If the P.A. needs me, I’m there in 5 or 10 minutes.  If they don’t, I get a free day of skiing in for my family and friends.  At the end of the day, I sign 4 or 5 charts.  Sure, it’s not much money (perhaps the equivalent of $100-200 a day in lift tickets and meal discounts), but it’s still an income.  When engaging in this type of work, look carefully at the liability issues.

The U.S. Internal Revenue Service categorizes income into three broad types, active income, passive income, and portfolio income.[1] It defines passive income as only coming from two sources: rental activity or "trade or business activities in which you do not materially participate."[2][3] Other financial and government institutions also recognize it as an income obtained as a result of capital growth or in relation to negative gearing. Passive income is usually taxable.

Taking some profits after a stellar bull run knowing a reversion to the mean will come at some point is not bad advice. Reinvesting during some of these low cycles of a secular bull market is also a good idea. Sticking your money under your mattress because of the fear of a doomsday scenario or forecasting the negative perfect storm from all of these bear prognosticators who continue to be wrong but like broken clocks may get to be right at a point in time of which no one knows for sure, is somewhat irrational. Much depends on your risk tolerance, time horizon, personal financial goals/objectives and another reason to be well diversified.
Since I knew I’d eventually be losing my day job income, I had to set realistic goals. There was no way I was going to make a six figure salary blogging and working online after only 2 years but I thought it might be possible to cover our monthly expenses for a few months while I took time off and looked for a new job. I already had some secondary sources of income and there were others that I specifically tried to build up.
However, not all of us use the time that we have wisely. In fact, we often squander it. We waste time with trivial pursuits, enthralled in things that don’t help to advance us towards our goals, or steeped in one bad habit after another, wallowing in self-pity and lamenting that there simply isn’t enough time in the day. In fact, you can tell a lot about a person by the way they use their time.
[we may view the] economic organization as a system of prize relations. Seen in the large, free enterprise is an organization of production and distribution in which individuals or family units get their real income, their "living," by selling productive power for money to "business units" or "enterprises", and buying with the money income thus obtained the direct goods and services which they consume. This view, it will be remembered, ignores for the sake of simplicity the fact that an appreciable fraction of the productive power in use at any time is not really employed in satisfying current wants but to make provision for increased want-satisfaction in the future; it treats society as it would be, or would tend to become, with progress absent, or in a “static” state.[13]
This is a fairly new concept to me still, but I now see that multiple streams of income are definitely far better than one high income stream as you point out. I now work part time as a hospitalist. That job provides our biggest income stream, but I also work part time for an opioid treatment center, which could easily turn into a full time position if I ever needed it (the owner has told me “you always have a job here”). Additionally, my wife works full time AND she runs an airbnb room out of our home which pulls in easily 1000/month. Those combined with our investments in stocks/bonds give us 5 sources of income. In the past we relied solely on my full time work and it stressed me out knowing that at any time something could happen that could take that away from us (as you listed above). Since I started doing these other things and we diversified our income, I feel much more at ease and relaxed.
Continuing residual income is the residual income that is expected over the long-term. Residual income will continue beyond a specified earnings horizon depending on the fortunes of the industry, as well as on the sustainability of a specific firm's competitive prospects over the longer-term. The projected rate at which residual income is expected to fade over the lifecycle of the firm is captured by a persistent factor, w, which is between zero and one

I have only dabbled in drop-shipping before when I had an eCommerce platform 6 years ago or so. I think it is something that you could do on the side, but you would want to do in depth research on the industry you want to get into before setting up shop. It may be a little less passive up front, but over time you could take your hands off the wheel.

The obvious way to earn a second income is to get a part-time job. If you are not currently working, this is an excellent way to start as it gives you the freedom and flexibility to start other passive income opportunities.  The other option is to simply work from home full time which frees up commute time so you can focus on building more income streams.
What I like about p2p investing on Lending Club is the website’s automated investing tool. You pick the criteria for loans in which you want to invest and the program does the rest. It will look for loans every day that meet those factors and automatically invest your money. It’s important because you’re collecting money on your loan investments every day so you want that money reinvested as soon as possible.
This is mostly passive once you have it all set up, but it does take a lot of work at the beginning. Real estate investing also requires occasional maintenance. Currently, we invest in a couple of rental properties and earn about $500 profit from each per month. You can read more about my rental properties at Money Smart Life - Money Tips for a Better Life: How and Why I Became a Landlord.

I’ve got a $185,000 CD generating 3% interest coming due. Although the return is low, it’s guaranteed. The CD gave me the confidence to investment more aggressively in risk over the years. My online interest income has come down since I aggressively deployed some capital at the beginning of the year and again during the February market correction. You’ll see these figures in my quarterly investment income update.

You can also participate in focus groups. Focus groups are hired to test products and give opinions before they reach the market. You would be a critical component of verifying they will have success and reach a high return on their investment. Surveys and focus groups are not ‘get rich quick’ schemes at all, but you can generate a few hundred a month.

The original version of Barbara Winter’s book, Making a Living Without a Job, came out in 1993, and in it, she recommended creating multiple "profit centers," as opposed to building a single income stream. Over ten years later,  Robert Allen, the real estate entrepreneur, also wrote a couple books promoting the idea of multiple streams of income. Back then, building more than one source of income was difficult, time-consuming, and expensive. Fortunately, things have changed today. The Internet has made it easier, faster, and more affordable to generate multiple income streams.

The legendary investor, Warren Buffett rightly said that if you cannot create a second source of income by the age of 45, then you have really done injustice to yourself. If you are in business or if you are an independent consultant there are multiple streams that you can consider. But what if you are employed and your existing contract does not allow you to take up other work? Also, your pressures at your workplace may be tight enough to leave you with little spare time to worry about how to create a second stream of income. That is when you have to make money work hard for you. Let us look at two such approaches.
The first application of residual income, the remaining money after debts are paid each month, is relevant when analyzing a person's financial status or ability to qualify for financing. The second application, the more widely recognized meaning of residual income, is money that is received on an on-going basis for work that is completed once. This form of income allows the recipient to generate revenue that is not based on time limits. Residual income is the foundation for wealth because it offers flexibility in earning and maximizing income. It also allows income to be generated long after the work has been done.