I don't know any 100% free, most of them offer a daily free limit or free trials:
semrush.com, ahrefs.com, kwfinder.com are the ones I have used and found very useful. I will recommend Google Keyword Planner. It gives most related keywords and its relevant searches. It becomes easy to finalise the keyword using this tool.
Well, a lot of keyword research tools aren't free. So when I first became interested in SEO writing. I gathered a few free tools that allow for 5-10 free searched keywords/phrases per day. All providing the statistical data (competition, popularity, number of backlinks, etc.) I need to choose which keywords to implement within my articles.