Keyword Planner uses Google's auto-complete APIs to generate ten thousand long-tail keywords from every single keyword you input. The tool also provides the monthly search volumes, CPC, Competition and 12 month trend data.

Answer The Public is a popular tool that extracts question based keywords from Google & presents them in a beautiful image. It's grown in popularity due to the fancy user interface which creates some great looking images to show off to your CEO at the next board meeting.

Keyword Finder is another new popular keyword tool used by a lot of marketers. Like others in the list, it uses multiple APIs to find the most number of hidden long-tail keywords for your businesses. They also provide the search volume, CPC and competition data.

Also Asked is a "People Also Ask" scraper that scapes Google for questions that your audience has. This tool has a beautiful UI as well that helps youunderstand how questions are topically grouped. It can be used to inform & inspire your content writers and to make sure that they are satisfying the searchers intent.

The final tool in our list, Keyword Generator is a simple tool that generates keywords for your online business. There are two levels of searching possible and while the Gold level takes a long time to execute, the tool is able to generate more keywords than any other tool in our list.