Multi-vendor marketplaces, like ThemeForest, can be very successful. Chose a niche and create a vendor website for it. Your marketplace could be anything, from a platform for local artists to sell their work on, to an online digital product store. Once set up, invite people in that industry to sell their products on your site. You take a percentage of their profits when items sell.
Whether it’s an important consumer application, a specialist app to solve a particular niche problem, or even a time-wasting game you can play on your phone, you can create a massively successful business if you build software that helps people. (Look at the rise of Slack—the team communication software that went from side project to billion-dollar company in just 2 years.)
I also participate in online surveys through the following websites, that work in the US and internationally (I use them all and I'm based in Brazil, for example) - Opinion Bureau, Myiyo, and Green Panthera. All of them have a variety of surveys and pay well. All you need to make money is to have 20 min or so to answer a few questions every day/week and have an active Paypal account so you can cash out.
In the past few years, craft marketplaces have popped up all over the Internet and offer a place where you can sell your handy work to those who appreciate handmade items. Etsy is the most popular marketplace out there. You can create your own web store on this site and list your handmade crafts, vintage items, and craft supplies. There are plenty of other sites on the web that cater to arts and crafts of all types. You can earn some extra money while enjoying your favorite hobbies.