Sell hair, plasma, or other body parts/fluids. Long, healthy, untreated hair can be sold for a variety of purposes (including high-end wig- and extension-making) and earn you anywhere from several hundred to several thousand dollars depending on color, health, and length. Plasma can be “donated” (for compensation) provided that you are old, healthy, and heavy enough to qualify. Sperm can be donated, but you usually have to know (and be able to document) a fair amount of information about your parents as well as your medical history to be eligible. But be wary; for example, donating eggs is often touted as a fast, high-profit medical procedure, but the process is actually requires that the participant undergo hormonal and medical treatment, receive regular checkups and ultrasounds, and abstain from sex and intoxicants, all of which take weeks or even months before the eggs are even eligible for removal. The removal process itself is invasive and lasts about 30 minutes. Consider your options very carefully before taking on anything like this.
For example, you might take photos and have them available for a fee at sites such as shutterstock.com, smugmug.com, 500px.com, or istockphoto.com. Similarly, you can create and upload designs at sites such as zazzle.com and cafepress.com, where people can buy them imprinted on shirts, mugs, and so on. Similarly, you might write an e-book and sell it online, perhaps via Amazon.com's direct publishing service.
Getting a raise is harder than getting a promotion. Think about it from your boss’s perspective, would you rather a) pay more money for the same service, or b) pay more money for additional responsibilities. Regardless, sometimes a raise is in order, especially if you have worked for several years without one. Check out Ramit Sethi’s guide on asking for a raise.
Hold a yard sale. If you have a yard or garage and plenty of items to sell, you can have a yard sale as early as tomorrow. By advertising your sale on local Facebook pages and Craigslist, you can also skip the paid newspaper ad and keep all of the profits for yourself. If you don’t have time to price everything, try asking patrons to “make an offer” or grouping similar items on tables with an advertised price (e.g. everything on this table is $5).
I’m a huge fan of blogging because I love writing and connecting with people all over the world. To give you an idea of what is possible with blogging, DollarSprout.com is part of a group of blogs that routinely brings in over $20,000 a month. While it takes time to build your blog up to that income level, you can get the initial set up done in under 30 minutes (no experience needed).