- https://ahrefs.com/ - "Rank higher and get more traffic." - Ahrefs is used by marketers from the world’s leading companies. Quality data is what we're known for. We’ve built advanced tech to crawl the web 24/7 to fill our multi-petabyte, multi-region database of keywords, backlinks and pages. - https://ahrefs.com/blog/keyword-difficulty/ - https://larryludwig.com/ahrefs-review/