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Semrush is a powerful and easy to use SEO, content marketing, competitor research, PPC and social media marketing from just one platform. With their free account you can do a limited number of keyword lookups and other site analytics per day to meet basic keyword research needs.
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Ubersuggest is a great tool for finding keyword variations that get the most traffic, offering a keyword ranking overview for individual sites as well as general keywords. You are allowed three lookups per day without registering, and a 7 day free trial is available.