I personally like this one for some spare cash. It does not take much to earn an extra $20 – $50 a week. Some claim that they are able to make a living by dedicating their time to different jobs at mTurk. You will have to go through a learning curve, but once you get comfortable, you will find plenty of good and interesting HITs (Human Intelligence Tasks) to work on.
You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.
Job well done, Satrap. The first thing I look for in a site like this are signs that the author is being paid to hype the companies. It’s very tough to find good information that isn’t stricken with bias. There were a couple of times during the article where I actually got excited thinking, ‘Hey, I could do that. I’d actually LOVE to do that.” That being said, I can’t help noticing that you refer to all of it as “extra” or “supplemental”income. Are you saying that with all of these options, sticking to one’s regular “day job” is required? In your experience, if someone really commits to doing this, is it possible to earn a decent living? Cubicles suck.
I see a comment made by Stella including some freelance sites. This does work. I use oDesk and it’s been a big help in bringing in a little extra money. I also agree with some of the other comments that mention people should think of or view more ideas of making extra money, because it really is possible to make extra money instead of just sitting around moping about not having money to pay the bills or buy something special.