There have been times in my life where I was tight on cash. This has happened for two reasons: either life happened, and I therefore wasn't able to meet my basic needs with the income I had, or I had some goals in mind that needed a quick cash infusion. In either circumstance, I searched out some ways to make extra money quickly. They weren't long-term or sustainable, but they were enough to help me get to where I wanted to be short-term.
Selling My Stuff
Oftentimes I've turned to Craigslist, the American equivalent of Gumtree. It's amazing how much stuff you can find around your house that you don't need or want, but is still sitting there taking up space. I compare prices of similar items currently selling on the site, and price competitively in order to get rid of it fast. Usually when I'm in hustle mode I get rid of a bunch of small stuff, but this also works if you suddenly don't need a larger item. Instead of setting it out with the trash, try listing it first. What seems useless to you may be worth a decent amount of money to someone else.
Mystery shopping was great when I first discovered it. I felt like an actor and a spy all in one. I went into stores and asked about products, secretly taking note of product knowledge and courtesy of employees as I pretended to be interested in buying them. I got paid decently for my time, and it served its purpose of filling my tills quickly. I didn't stay with it long-term, though. It was a pain logging into the system at just the right time to get the best compensated shops, and what was leftover were stores that were either terribly inconvenient or terribly hard for me to go on pretending about.
Participating as a subject in a medical study can be very lucrative. Sometimes they want to study the effects of medication on humans, but sometimes they just want to see what makes certain people sleep poorly, or run a psychological questionnaire on a targeted demographic. All of the studies I've participated in have offered a smaller sum just to come in for the initial interview to see if you're a good fit for their study, but they also allow you to see if the study is a good fit for you. If I decided I wasn't comfortable with what they wanted to test out on my body, I still had the option to back out while still being rewarded with the pay for the initial interview and my time.
Turking and SurveysFor a short time when money was really tight, I tried turking and taking surveys for a little while. Amazon Turk is a program that has you do menial tasks that only human brains can accomplish; computers are as of yet incapable. So I looked at pictures and decided if they looked pretty. I captioned a video clip or two. (Some voice recognition software out there might be amazing, but most of what's widely used is still far from being acceptable.) I even wrote a few articles. It ended up being time consuming, and I was usually getting paid pennies or dimes for my effort. I quickly gave this one up.
I stuck with surveys a little bit longer. I was making a few dollars for each one I did, and the payout for the company I was using was very low. I felt great about the $8 thresh hold, but I started getting qualified for fewer and fewer surveys, and started questioning if 20 minutes of my time was worth $2.