AnswerThePublic
Answer the Public describes itself as a "Search listening tool for market, customer & content research." It's a way to see what search terms people enter about a particular topic or brand. If you're wondering what questions people have about a subject, this would give you data about what they search for online. You can only do one search per day for free; otherwise it's $99/month for unlimited searches (which only makes sense if you're doing a LOT of market research).