Scraping Capterra and Trustradius's reviews is quite useful (I have a script for it).
Otherwise mining quora with QApop> can give you a nice idea about what people are asking for.
SparkToro is definitely a super tool/
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AnswerThePublic, Google (Search & Trends), Semrush, Wappalyzer, Storeleads, KnoCommerce, Gartner, and a few others depending on what we’re looking for.