However, under certain circumstances, payment for personal services performed in the United States is not considered income from sources within the United States. For example, personal services performed by an independent nonresident alien contractor specifically exempted by a tax treaty. For more examples, see the Pay for Personal Service section in Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities.
It’s a (mostly) short term, higher risk, higher reward place to invest cash that has a low correlation with the stock market, but is far more passive than buying and managing properties, has more opportunity for diversification than private placements (minimums of 5-10K, rather than 100K), and most of the equity offerings (and all of the debt offerings) provide monthly or quarterly incomes. Unlike a REIT, you can choose exactly which projects you wish to invest in.
Sell Digital Files on Etsy – I’ve been into home décor lately and I had to turn to Etsy to find exactly what I wanted. I ended up purchasing digital files of the artwork I wanted printed out! The seller had made a bunch of wall art, digitized, and listed it on Etsy for instant download. There are other popular digital files on Etsy as well such as monthly planners. If you’re into graphic design this could be an amazing passive income idea for you.
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I like to think about having an additional source of income as being sort of like an insurance policy. I interviewed one young woman who taught aerobics classes in the evenings after spending her days working as a videographer for news organizations. When she was unexpectedly laid off from her day job, she quickly doubled her aerobics teaching load and was able to keep herself afloat until she found a new full-time job. Some more examples of income multi-streaming are a teacher I know who sells used baby clothes out of her home and a young couple who run a small Internet business on the weekends. Just like these people, you can also pick a profitable project or create a business that matches your lifestyle. 
Given the growth in the sharing economy, your junk can start to pay for itself. For example, if you have some awesome vintage furniture inherited from your grandmother sitting in a storage unit, you can rent this out to photographers for their “styled shoots” which are becoming all the rage. If your furniture is more modern but you still can’t bear to get rid of it – perhaps a home stager will be interested.
Thanks for asking. gives a good summary of the definition I use. But in brief, it’s income that isn’t proportional to the time you physically put into acquiring it. It doesn’t mean it’s not without work or effort. It’s just that most of the work is done up front and it continues to pay off long after that initial effort. Real estate fits into that box. There’s definitely a spectrum but compared to what we do as doctors, where our compensation is directly linked to our time, most of these things are quite passive.
And, let’s face it; not everyone wants to sling body wraps or nutrition shakes to their family and friends. That’s why offering a service is often one of the best, and least uncomfortable, ways to earn side income doing something you love. Maybe you want to clean houses or rake leaves or help people file their taxes. It’s totally up to you, and that’s the beauty of this option.