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!
Betterment – Betterment was the first robo-advisor to launch, almost ten years ago. They’ve automated the entire investing process, so all you have to do is watch your portfolio of assets grow (over the long run, of course). They do charge a .25% annual fee of your account total, so if you’ve got $100,000 that’s being managed by Betterment, you’ll pay just over $20 per month.

To model Viral Cycle Time’s effect on growth, I searched the web, high and low, looking for a pre-defined formula. To my great surprise, there was no formula that I could find that correctly calculated customer growth, and showed the impact of Viral Cycle Time. What was also surprising, was that I did find several blogs showing formulae for viral growth, but in every case, they appeared to make the same mistake, which was assuming that the entire customer base would continue sending out invitations for every cycle. So I collaborated with my partner, Stan Reiss, who turns out to be a whole lot smarter than I am, and he helped me develop the fomulae that are used in the more sophisticated model for viral growth below:

The term viral strategy was first used in marketing in 1995, in a pre-digital marketing era, by a strategy team at Chiat/Day advertising in LA (now TBWA LA) for the launch of the first PlayStation for Sony Computer Entertainment. Born from a need to combat huge target cynicism the insight was that people reject things pushed at them but seek out things that elude them. Chiat/Day created a 'stealth' campaign to go after influencers/opinion leaders, using street teams for the first time in brand marketing and layered an intricate omni-channel web of info and intrigue. Insiders picked up on it and spread the word. Within 6 months PlayStation was number one in its category—Sony's most successful launch in history.

If you have a blog or some other property that has visitors, you might profit passively via affiliate advertising. For example, imagine that you write a blog about movies. You might review some books about movies, and then link to them on Amazon so you get a cut of the purchase price when anyone buys books through the links. If you blog about hiking, you might promote some hiking gear you recommend on the blog, again generating passive income if anyone buys any of it.
1) If your property is not under rent control, you have the ability to raise rent to market prices with proper warning. In SF, I have to give tenants a one month warning for up to a 10% increase and a two month warning for up to 60%. Rent control limits to an inflation index, usually around 2% a year, which is why initial pricing and tenant turnover is important for better profitability.
ie first you need to haul ass and do something crazy, eg write a quality 20,000 word ebook (insanely not passive hahahah), but then you get to sit back and enjoy seeing PayPal sale messages pop up on your iPhone each morning as sale after sale after sale is made…on an ongoing basis and without any additional work. That’s some seriously Pina Colada flavored passive goodness!

A REIT is a company that owns, operates or finances real estate and allows anyone to invest in portfolios of real estate assets, the same way as stocks: you can purchase individual stock or exchange-traded fund (ETF). You can then earn a share of the income produced through the real estate investment without having to own, manage or finance a property.
Remember the HillTop ad that Coca-Cola did? If not, perhaps the lyrics will spark a memory - "I'd Like to Buy the World a Coke."  Is the song stuck in your head?  This jingle had the whole world singing, and if that weren't enough, it was recently revisited on the Mad Man series that had everyone humming it again.  The song spoke of unity and tolerance, and as consumers, we gravitated to it and shared it.
When you invest in a dividend-paying stock, you are buying a share of the company and you literally become part-owner of that business. As the company grows and generates extra cash that it doesn’t necessarily want to re-invest, it might decide to return some of the extra cash to the shareholders in the form of dividends. And because you own a fraction of the company, you will receive a portion of the cash!
The most viral products are those that only work if they are shared. For example, Skype only worked in the early days if you got your friends on to Skype, otherwise you had no way to call them. If you have an application today, think about how you can make it social, where it would work better by sharing data with friends/co-workers. That provides a great incentive for customers to invite their friends/colleagues to use the application.
And while real estate is an excellent option, it does require a significant initial investment, so whether or not this passive income stream is right for you depends on your current financial situation. You might be better off starting with an investment strategy where you can build funds until you have a big enough sum to get involved in real estate.
Venture debt ($12,240 a year): The first venture-debt fund has returned almost all my initial capital, so I decided to invest $200,000 in the second fund. I took a risk investing $150,000 in my friend's first fund, so I'm hoping there's less risk in the second fund, given he has four more years of experience on top of his 12-plus years of experience running a venture-debt portfolio for another company.
The disadvantages to consider with viral marketing include the nuisance factor. Due to the large amount of email people receive daily, viral marketing messages may be viewed as spam. Another consideration is negative marketing buzz. A message may not be construed as intended, and could be discussed in a negative manner. Viral marketing can also be hard to measure. 
Now I’ve been using Swagbucks for a while and have found the money works out to just under $2 an hour so this isn’t something that’s going to make you rich. You’d have to work 2,500 hours to make $5,000 so that’s about three and a half months, non-stop. The thing with Swagbucks though is you can do it when you’re doing something else so I flip through surveys and other stuff while I’m cooking dinner or flipping channels.
Viral advertising is personal and, while coming from an identified sponsor, it does not mean businesses pay for its distribution.[6] Most of the well-known viral ads circulating online are ads paid by a sponsor company, launched either on their own platform (company webpage or social media profile) or on social media websites such as YouTube.[7] Consumers receive the page link from a social media network or copy the entire ad from a website and pass it along through e-mail or posting it on a blog, webpage or social media profile. Viral marketing may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, text messages, email messages, or web pages. The most commonly utilized transmission vehicles for viral messages include: pass-along based, incentive based, trendy based, and undercover based. However, the creative nature of viral marketing enables an "endless amount of potential forms and vehicles the messages can utilize for transmission", including mobile devices.[8]