Everyone already knows about Amazon and it's become one of the first places people visit if they need something though. No one needs to introduce you to Amazon and Amazon really doesn't need those YouTube links and what not to drive traffic to them. It would've made sense when it was just launching, but I see little reason for Amazon to be paying for an affiliate program when they're one of the largest (if not the largest) ecommerce sites out there.
Whether you know how to flawlessly apply eye makeup or build a wooden shelving unit, you can create an online course or video for others to follow. Websites like Udemy and Teachable allow you to build a course, such as how to learn a new language or write a cover letter. Once you build your course and set the price, there’s little work to be done. You’ll receive residual income from each person who signs up to take your course.
P2P lending is the practice of loaning money to borrowers who typically don’t qualify for traditional loans. As the lender you have the ability to choose the borrowers and are able to spread your investment amount out to mitigate your risk. The most popular peer to peer lending platform is Lending Club. You can read our full lending club review here: Lending Club Review.
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 MoneySmartLife.com: How and Why I Became a Landlord.

Successful YouTubers choose a theme for their channel—ranging from humorous commentary to gardening. As you acquire subscribers to your channel, your number of views becomes more consistent, which in turn grows your residual income. While you’ll need to produce regular videos to maintain your following, a couple hours of filming and editing pales in comparison to the revenue you can generate with minimum effort.

If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more. 
I think also a very good way to earn a nice passive income is investing in Cryptocurrency, especially in Masternode Cryptocurrencies, which provide a passive income in coins, also those carefully picked coins grow in value, so it’s a double gain! And a great coin to invest in at the moment is GINCOIN, which is the fuel for a really succesful project. Find more at GINCOIN Website: https://gincoin.io/ 😉
Good suggestions. I have many of these. One word about the “app” idea. I had a great idea related to personal taxes that I tried to get off the ground with my accountant as a partner. I would say it’s difficult to do this unless you have a coder on your team. Hiring someone is not really viable financially unless the app is simple. When we finally got the quote for a coder to write what we wanted (and after doing lots of mock ups ourselves and getting a demo for investors) the estimate was about 750k just to really get started.

Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.