Answer the Public is a helpful (and free!) long-tail keyword research tool. Type in a keyword, and it generates common questions asked about that keyword. It will give you a good direction for your content.
SEMrush allows you to search for keywords and find their rankings in all the major search engines. You can also search for keywords based on geographic location. While this is a paid service, they do offer a free 14-day trial.
KWFinder has a quick search feature that can generate hundreds of low-competition keywords relevant to your site. It also pairs each keyword with an SEO difficulty score, which will help you optimize your SEO strategy.
This keyword tool is very easy to use. Just type. a keyword, target market, and country into the search feature; Ubersuggest will show you a list of long-tail keywords and topics you should cover in order to increase your search rankings.
The key feature of SEcockpit is its rank tracker, which allows you to evaluate the completeness of keywords. Try it first with an easy keyword, and see what happens. This is a paid service, but you can start with a free trial.
Simple, powerful, and free - that's what makes Instakeywords one of the top plugins for long-tail keyword research. All you need to do is enter a "seed" keyword to generate hundreds of ideas for SEO-friendly content.