The doctor or lawyer, for instance, could use her or his income to invest in a medical start-up or buy shares of medical companies he understands such as Johnson & Johnson. Over time, the nature of compounding, dollar cost averaging, and reinvesting dividends can result in her or his portfolio generating substantial passive income. The downside is that it can take decades to achieve enough to truly improve your standard of living. However, it is still the surest path to wealth based on the historical performance of business ownership and stocks.
When money is lent to a partnership or S-corporation acting as a pass-through entity (essentially a business that is designed to reduce the effects of double taxation) by that entity’s owner, the interest income on that loan to the portfolio income can qualify as passive income. According to the IRS's Passive Activity and At-Risk Rules: "Certain self-charged interest income or deductions may be treated as passive activity gross income or passive activity deductions if the loan proceeds are used in a passive activity."
Better yet, you can even upload your own book to one of the world’s largest book sellers: Amazon. With Amazon self-publishing, you set the price, retain the rights to your book, and get access to Amazon’s massive audience. For every sale, you keep 70% with Amazon taking the remainder as a fee. If you want to get started, check out Leslie Samuel's great guide to selling eBooks online or follow Tara Gentile on CreativeLive as she shows you how to use your existing body of work to write an eBook within the next week. Who knows, you might just write one of the best business books of this year!

ClickBank allows you to join for free, and the approval process is virtually automatic, so it’s a great choice for people entering the affiliated game for the first time. ClickBank has a ton of information, including FAQs, walk-throughs, and videos available, so the barrier to entry is quite low. There’s also a (paid) program called ClickBank University with courses and assistance from experienced marketers.
If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.
Etsy: While Etsy's popularity has declined recently, it's still a great resource for selling handmade items online. No need for complex ecommerce sites or merchant accounts or any sort of automation. The company takes a commission of every sale and charges a small listing fee per item. But many still use Etsy as their primary source of income. The best part is that you can also sell digital products on here such as poster designs. 
Well said Illidi. There are more ways than ever, mostly because of the internet. I think the secret is to put out a product or service that’s unique. Not that you have to invent a whole new business, but take an existing business concept, and add something unique to it. Because you’re absolutely right, everyone is getting into the game, making it harder to succeed.
I started a blog which I plan to monetize only through affiliate marketing and my own products, no ads. I’ve been working on building an audience for my blog, for about 1 year and a half, many people think is maybe too much time, but I just want to make sure that I build enough trust with my readers before I start to try to make them buy something.
Test websites. Remote usability testing means getting paid to navigate a website for the first time and giving feedback to the website owner. Most tests take approximately 15 minutes, and you can get paid up to $10 for each test. A test involves performing a scenario on the client’s website and recording yourself doing it. For example, you might be asked to go through the process of selecting and purchasing an item on a retailer’s website.[1]
Build up a following on your Instagram account and you could quickly be making extra money online. Major brands, gear companies, and even startups are willing to shell out $500-$5,000+ per post to get in front of your audience. While it’s getting harder and harder to build a massive Instagram audience, if you already have a solid niche and are posting quality content regularly with a great camera for taking Instagram photos, with a few small tweaks you can make yourself an influencer. Check out this awesome article from Shopify on how to build and grow your Instagram following to get started.
– Project Payday is one of those sites that has testimonials of people who have earned thousands of dollars by getting paid to get trial offers. I’m not saying you’ll earn thousands, but it is legit and you can earn some extra cash. They assume that by paying you to do a free trial, you’ll either like the product and purchase it, or forget to cancel the trial and get charged for it. If you can keep track and cancel before you get charged (if you don’t want the product), then this is a great site for making some money.
File-Sharing: Web sites that host directories of music, movies, games and other software. Users upload content to file-hosting sites and then post descriptions of the material and their download links on directory sites. Uploaders are paid by the file-hosting sites based on the number of times their files are downloaded. The file-hosting sites sell premium download access to the files to the general public. The websites that host the directory services sell advertising and do not host the files themselves.
AWIN is probably best for experienced affiliates who can hit the ground running without a lot of guidance or feedback from the network. There is a $5 fee charged to apply to become an affiliate, but if you’re approved, the $5 will be added to your account. If your application is denied, however, you will lose the $5 fee. AWIN operates globally, but it is most heavily concentrated on British and EU merchants.

If you have a propensity for writing and you can slay with your prose, consider writing an ebook. While the market has certainly become saturated as of late, books that help teach people about a technical topic still sell extremely well. This is a great source of passive income but does require a large amount of effort at the outset before any money is generated.