Upwork: This website offers a great marketplace for selling just about any professional service. You don't need a merchant account, website of your own or anything else for that matter. All you need to do is be able to provide a high-quality service at a reasonable price. But be informed, you will have to compete with many others that are constantly bidding on open jobs.
If you live in an area where Uber or Lyft operate, why not become a driver? If you're looking to make some short-term cash, you can definitely rake it in by working for one of these popular car-hire apps. As long as your vehicle fits within the specifications of their program, and you have a clean license, you could do this on the side, especially if you're in a crunch for cash.
Take surveys. You can earn $50 to $100 per month in cash and products by taking online surveys. Find survey sites by searching for “paid survey sites” online. Sign up for several survey sites to increase your chances of being selected for higher-paying surveys. Register with an email address, and check your email often so you can respond quickly to survey offers.
However, like anything else truly worthwhile, apps require a significant investment of your time or money upfront. If you don't have the skills, then you have to hire someone who can assist you in creating a great app. But first you need to come up with an idea that will sell. Do the proper market research and analytics to come up with the right app.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.