Instead of going to google ” keyword tool”, here is a quicker list that I shown as a real tool to use:
ads.google.com – keyword planner
generate LSI keywords — what is LSI keyword ? Introduction Article here.
spyfu – search competitors keywords
instakeywords — free keyword tool
wincher — tracking keywords rank in google
It has 3 major functions: explore keywords, keyword rank analysis, keyword content enhancing assistant. It shows details how difficult is your keyword, and back-links difficulties as well. For ranking analysis, it is a good indicator for content/article keywords focus.