If you have extra time with you over weekends or after your job, you can freelance and take small projects. There are many websites like elance.com , odesk.com or freelancer.com where you can earn money by working on projects. There are thousands of people in India who are working full time on these kind of freelancing websites and believe me, they are earning in lakhs. Look at it as a serious way of making money and not just a time pass activity.

2) Find Out What You Are Good At. Everybody is good at something, be it investing, playing an instrument, playing a sport, communications, writing, art, dance and so forth. You should also list several things that interest you most. If you can combine your interest plus expertise, you should be able to monetize your skills. A tennis player can teach tennis for $65 an hour. A writer can pen her first novel. A finance buff can invest in stocks. A singer can record his first song. The more interests and skills you have, the higher chance you can create something that can provide passive income down the road.
What’s also really important to realize here is that when I took the exam I was teaching people to study for, I didn’t get a perfect score. In fact, I didn’t even get close to a perfect score. I passed. But I also knew a lot about this exam—way more than somebody who was just getting started diving into studying for it. And it was because of that, because I was just a few steps ahead of them, that they trusted me to help them with that information. To support this, I provided a lot of great free value to help them along the way. I engaged in conversations and interacted in comments sections and on forums. Most of all, I just really cared about those people, because I struggled big-time with that exam myself.
The original version of Barbara Winter’s book, Making a Living Without a Job, came out in 1993, and in it, she recommended creating multiple "profit centers," as opposed to building a single income stream. Over ten years later,  Robert Allen, the real estate entrepreneur, also wrote a couple books promoting the idea of multiple streams of income. Back then, building more than one source of income was difficult, time-consuming, and expensive. Fortunately, things have changed today. The Internet has made it easier, faster, and more affordable to generate multiple income streams.
That is a nice list of passive income sources. Actually, the most up-to-date list of dividend growth stocks is the list of dividend champions, maintained by Dave Fish. The list of dividend aristocrats is incomplete at best. For example, the dividend champions list has over 100 companies that have managed to increase dividends each year for at least 25 years in a row. The list of dividend aristocrats has no more than 50 – 60.
You are also free to choose a fund that is based on any index that you want. For example, there are index funds set up for just about every market sector there is — energy, precious metals, banking, emerging markets — you name it. All you have to do is decide that you want to participate, then contribute money and sit back and relax. Your stock portfolio will then be on automatic pilot.
This has become a very fast growing business. One can make videos in an area that you like — it can be in music, tutorials in any field such as makeup tutorials, drawing tutorials, vlogging and many more like these, reviewing, unboxing experience, reactions, comedy etc., anything which you think people are going to like. Upload these video on YouTube. One can then attach Google AdSense code to the videos. This will overlay the videos with automatic ads. When the viewers on your videos will click on those ads, one will earn money from AdSense.
As for me, I started focusing on passive income last year, but have owned rentals for 5 years. $25k now outside retirement accounts in mostly real estate. Looking to invest another $500k cash into real estate to get about $65k, and then 1031 under performers next year to hopefully boost that a bit higher. Heavy in real estate, but feels lower risk than the stock market to me if you have cashflowing properties. Real estate is inflation adjusted, and built in cashflow raise when the loan pays off.

One of the things I'm surprised your article doesn't mention is the tax advantages of this type of investment. The depreciation and rehab costs (purchasing distressed properties) can be huge deductions to ones income taxes, which none of the others have. Then, along with the appreciation of real estate, this passive income investment outperforms the notion of maxing out my 401k as well.

However, until we get another reset in valuations (I’m calculating a 40% to 50% correction is justified ), I’ve moved largely to the sidelines. Beginning in July 2013, I began slowly reducing equity exposure and am now sitting firm at 40% with the balance in various forms of 5 yr cd’s and short duration bonds. This is down from over 60% when I ramped up to take advantage of the March 2009 lows.
Blogging – I guess you could say I’m a professional personal finance blogger since I own two sites and I’m making decent money every month. The income started off slow but has been consistently increasing. It’s not as much as I make with my day job but my best blogging month was equal to about one paycheck at my old day job.  While I had to learn how to set up and use WordPress myself, you can learn how to blog and make money online at StartABlog123.com.
What I Do: I’ve set up multiple investment accounts outside my main operations bank that deals with working capital e.g checking, paying bills. By transferring my money to a couple brokerage accounts and two other banks as soon as it hits my main bank I no longer have temptation to spend on frivolous things. As a result, I can wake up 10 years later and reap the rewards of compounding. My 401(k) is the best example where constant contributions over 18 years has grown to over $500,000 without any savings pain given it just became a part of life. Real estate is also a fantastic asset class for the long term. It’s fantastic to enjoy your home, pay down your mortgage each month, and end up with a paid off asset that has likely appreciated during your time of ownership. 

Doesn’t it sound awe-inspiring to have more than one income source? You already have one source of work with a steady flow of income and then you are creating more and more work for you with more income for you. Who does not want to have lots of money in their bank accounts floating all around? For the person who values financial security and their ultimate dream is financial freedom, creating more than one source of income becomes a necessity not just desire.
You might be thinking one job makes you exhausted by the time you come home from work, how can you create second, third, fourth one and work so efficiently for them. You have to find out ways of creating multiple sources that are suitable for you and can add steady flow of income in your monthly salary basket. It could be as low as INR. 100/- per month but has to be steady.
For example my business is a LLC taxed as a S corp. I am active in it and my wife is not. She owns half the company because she fronted the money to start the company (but is not active at all in the business). I get paid a W2 salary for my work I put into it and any profits are distributed to my Wife and I as “dividends”. However the dividends are still taxed as active income at the higher tax rates.
I also noticed that in your passive income chart at the bottom that you don’t include your internet income other than sales from your book. Is there a reason for that? Do you not consider is passive because you are actively blogging all the time to create it? Or do you just not want readers to know how much money you generate from blogging activities?
Sonme months back, I heard the founder of Tastykhana.in in a TIE talk, where he shared that when they desperately needed some money in the starting years of their business, that time – one of the employee of an IT company put in Rs 1 lac in their business and within a year or two, he got back 20 lacs return through an exit option when they got funding later (it was something like this, if not exact)
After these tenants move out, I'm thinking of just keeping the rental empty with furniture. It sounds stupid to give up $4,200 a month, but I really hate dealing with the homeowner association, move-in/move-out rules, and maintenance issues. Given that the condo doesn't have a mortgage and I have to pay taxes on some of the rental income, I'm not giving up that much. The condo can be a place for my sister, parents, or in-laws to crash when they want to stay in SF for longer than a week or two.

The obvious way to earn a second income is to get a part-time job. If you are not currently working, this is an excellent way to start as it gives you the freedom and flexibility to start other passive income opportunities.  The other option is to simply work from home full time which frees up commute time so you can focus on building more income streams.

Hannah UnplannedFinance That’s a good way of putting it, Hannah. I would say it’s better to get into the investing game before an emergency comes up, of course, so that your investment income is a bit more stable (hopefully). They’re good parts to include in your whole plan, which should be a balanced one. I was coming at this from more of a “how to prepare” perspective.
I have several streams, which is really nice because you never know when one will dry up. I think I will be losing one of my contract optometry positions later this year. It sucks because I like the job, but it won’t be financial ruin. Even if you have a secure job, the side income does allow you to meet goals faster. I don’t think I’d ever go back to relying on one job, even if it was awesome.
Education is one sector which is totally immune from recession. I wrote an article on education sector Education – Problem or Solution There are many opportunities in education sector to earn Second Income. You can work part time or during weekends. Like foreign countries, in India also there is a demand for online tutors. You can earn handsomely as an online Tutor.
Indian Railways is the fourth-largest rail network in the world, with a track length of 114,500 kilometres (71,100 mi) and 7,172 stations. This government-owned-and-operated railway network carried an average of 23 million passengers a day, and over a billion tonnes of freight in 2013.[238] India has a coastline of 7,500 kilometres (4,700 mi) with 13 major ports and 60 operational non-major ports, which together handle 95% of the country's external trade by volume and 70% by value (most of the remainder handled by air).[239] Nhava Sheva, Mumbai is the largest public port, while Mundra is the largest private sea port.[240] The airport infrastructure of India includes 125 airports,[241] of which 66 airports are licensed to handle both passengers and cargo.[242]
If you want to really start tracking your finances, and I mean not just your spending but your investing (that's where wealth is built), give Personal Capital a look. They will give you a $20 Amazon gift card if you link up an investment account that has $1,000+. No strings. It's a cornerstone of my financial system and I think you owe yourself a look. 100% free too.