1) SEMrush - I believe that among all the 3rd party software, SEMrush has the largest keyword database. Their search volume data is pretty accurate and aligns with the Google keyword planner. Also, based on the type of content that needs to be produced (i.e. informational, transactional, etc.), one can utilize different filtering options available in it.
But if you’re serious about content and inbound marketing, and you want to create great industry-related blog posts that lead to more traffic, leads, and sales–then you should at least give it a test drive (assuming you need content ideas). Like I said, it comes with an excellent guarantee, and you can visit the site here or click the image below to watch more videos regarding this impressive tool.
TIP: A really good strategy for increasing your search engine rankings (and maybe even getting a featured snippet), is to pick a number of popular questions, and answer them in your content. You can do this in the form of a ‘Question & Answer’ section or maybe ‘FAQs’. Just pick half a dozen or so questions, and list them, together with a short answer.
Repeat this exercise for as many topic buckets as you have. And remember, if you're having trouble coming up with relevant search terms, you can always head on over to your employees on the front lines -- like Sales or Services -- and ask them what types of terms their prospects and customers use, or common questions they have. Those are often great starting points for keyword research.
It's wonderful to deal with keywords that have 5,000 searches a day, or even 500 searches a day, but in reality, these popular search terms actually make up less than 30% of the searches performed on the web. The remaining 70% lie in what's called the "long tail" of search. The long tail contains hundreds of millions of unique searches that might be conducted a few times in any given day, but, when taken together, comprise the majority of the world's search volume.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.