Respondent allows you to search for both online and in-person surveys and focus groups. They pay out via PayPal and encourage international registration.
The payment amounts for these research studies can vary a lot, but even the lowest paying ones are nothing to chuff at. For example, there’s currently a survey about IoT devices from Microsoft that pays $15 and takes approximately 10 minutes.
Setting aside 4-5 hours per week to apply to and complete online surveys can net you over $500 per month.