[mbym]

Prolific (http://prolific.ac/) is designed specifically for science research, but I've used used it for two different use cases: 1. Product research, quickly testing appetite for an idea. 2. Micro tasks (a "survey" with at least one participant, where basically the only question was "have you done the task (on external page)?" The task in question was editing a public transcript, so confidentiality wasn't even a concern.

Each time I was impressed with the quality of the responses. In the optional open-ended text fields for the research survey, in particular, I was impressed with how people took time to free-write their thoughts, rather than just rushing on to the next question / to complete the survey. Point is I do think you get what you pay for...

From my experience, it seems Prolific's ethical approach (e.g., insisting on decent minimum payment rates) leads to overall higher quality of participants as well as responses (vs. race-to-the-bottom micro-task platforms like MTurk). [Disclaimer: I'm friends with the founders -- so I'm quite familiar with the amount of resources devoted to ensuring truthful/quality responses.]