To test content in more depth than an A/B trial permits
Better control over 'who is participating' and how much attention they are paying
Things more towards 'basic background research'
Closer to a 'representative sample'
Prolific participant recruitment: Created specifically for (academic research). Our impression is that this is among the highest quality panels, although there is some controversy over this.
CloudResearch: CR approved Mturk
CloudResearch: Prime Panels
Crowdflower Positly: https://www.positly.com/ Qualtrics (panel) Lucid
Dyndata