Fiverr: Israeli-based Fivver was started in 2010 by Shal Wininger and Micha Kaufam. It's a great resource for selling just about any service online. You can offer gigs as low as $5 but also get paid much more for upgrades and add-ons. There are plenty of providers earning 6 figures on Fiverr so it's definitely a worthwhile cause for generating a healthy income. Just ensure that you provide some serious value.
Drive for Uber or Lyft: If you're in a locale where you can find Uber or Lyft (or even one of the many competitors around the world such as China's Didi), you could easily make a respectable income. The hours are flexible and you can work as you see fit, making it perfect even if you currently have full-time employment but are looking to make some money on the side.
Websites like Survey Junkie will pay you a decent chunk of change for the low-maintenance, borderline mindless task of completing surveys. Companies want to understand consumers better, and one way they do that is by compensating survey-takers (a.k.a. you). Most surveys pay between $0.50 and $1.25, and many of them take less than 5 minutes to do. You can read our full Survey Junkie review for more info.