One of the most well known ways of generating residual income is through ownership of stock in a publicly-traded company. Residual income streams can also include things like royalties from the sale of a book or song. Today, there are also several online platforms like Lending Club or Prosper that allow you to invest in personal or small business loans.
Here's another example. If a person owns apartments that are rented out at a profit, they earn money each month without working a specific number of hours. If the owner earns $100/month net income from each apartment, the determining factor is how many apartments they own, not how many hours they work. Therefore, they may own 1 apartment and make $100 net income each month, or they could own 500 apartments and make $50,000 ($100 per apartment) each month.

Residual income is different from a salary, or linear income, which is paid out strictly based on the number of hours a person works. Someone who works on a salary is often said to work “paycheck to paycheck.” This is because he pays all of his bills with his first paycheck and then must wait until he gets paid again to have more money. Ideally, someone will work hard building up a business so that he can enjoy the residual income once his goals have been met. Then he can work on additional projects while still earning money from his business.

Writing an e-book is very popular among bloggers, as many have noted that “it's just a bunch of blog posts put together!” You will not only have to make an investment of time and energy to create the e-book, but market it correctly. However, if marketed correctly (through blogging affiliates in your niche, for example), you could have residual sales that last a very long time.
As doctors, we most likely fall into the first category. We may be rich in the sense that our salaries are in the higher economic range, however, because of our expenses (houses, cars, student loan debt, private school tuition, practice overhead, etc.) and sometimes poor decisions, we have a tough time accumulating any real wealth. We’re also handicapped by the fact that we start along this financial journey relatively later in life.
Whether you choose to invest in just one of these modern REITs or both, keep in mind that since they’re private funds and not stocks, you won’t be able to easily liquidate your investment and access your cash right away. Depending on your investment, plan to see your money tied up for anywhere from six months to five years. However, you’ll most likely still receive monthly or quarterly payments, depending on which investment opportunity you select.
For example, you can design digital products, like animal clipart or a downloadable wedding poem that could be printed by the customer. Your design could be resold thousands of times without needing to create each item or ship it. Once you create your digital product and list it on a site like Etsy or Ebay, the revenue flows in with little oversight.
One word of advice, and something I intend to do once I have the money saved up, is to build or buy out property that can support apartments or townhomes. One tough mistake some people make is buying a pair of homes to rent out and they get a nice $2,000-$3,000 a month but that’s it. Buying a house is expensive and the rental prices keep lower income families from potentially coming to you with their money to rent. If you have an acre to work with (more or less is OK too) you should be talking to a contractor to build apartments or townhomes. You will make a little less per unit BUT your audience grows significantly because now you can have college students, single parents, older folks, etc. all able to afford your rental units AND instead of capturing one $1,000-$1,500 a month payments, you can probably charge $700 a month per unit (or more, depending on the market) and build maybe 3, 4, 5, 10 units for the price of a home or two and now you’re making something like $2,100-$10,000 a month. It all depends on what you have to invest but if you’ve got $250,000+ I’d highly suggest you talk to a bank/investor that can get you in touch with a good contractor to build on a property and get permits and take out a matching $250,000 loan (I’ve read that $500,000 is plenty to build a good amount of apartments to start) and you can fill up your apartments and make a killing every month. You’ll have more tenants to deal with but if you’re competitive with your pricing you won’t have a hard time keeping tenants or replacing them.