While this one isn’t necessarily a keyword research tool, it will give you valuable insight into how the keywords you’ve been optimizing for so far are actually performing for you. You might realize that you could start ranking for more keywords with more difficult competition, for example, or that only keywords on certain subjects are working for you.
1) Ahrefs to quickly see “the big picture” when it comes to any keyword I'm researching. I can instantly see the top holders in the SERPs. I then immediately take the top holders list and go check out their sites. I need to make sure I can beat them content-wise, otherwise I will search for another keyword to try and rank for, or perhaps go down the long-tail route. The Ahrefs tool and data quality get better and better every year. It's one of my favorite tools.
How do I know that the KC score measures competition accurately? When I first started using the KC feature – I didn’t know for sure. But I’ve been using it since December of 2012. I’ve published literally hundreds of different articles (in different niches) around keywords with low to medium KC scores and I have a pretty good feel for the accuracy. More than 5 years of successfully ranking for low competition keywords has convinced me to completely trust the KC scores that are calculated within Long Tail Pro.
One possible problem is that although the Keyword Planner has some cool new features (including integration of the Google Traffic Estimator, which will be retired too), as of this writing, the indispensable Exact Match and Phrase Match features are nowhere to be found! Whole books will become useless without these, so I hope Google will eventually port them to the Keyword Planner. Nor do you see the “Include specific content” option, which is a life-saver for the adult industry, which spends the most on Google PPC.

Google Suggest isn’t exactly a tool, but I’ve found that it can be useful for identifying potential keywords. As you type a query on Google.com, Google Suggest recommends search queries based on other users’ search activities. These searches are algorithmically determined based on a number of purely objective factors (including popularity of search terms) without human intervention. The Suggest dataset is updated frequently to offer search queries that seem to be trending upwards. This feature is largely one of the reasons that you may see repeat traffic of seemingly long tail keywords. By identifying these long tail keywords and optimizing for them, marketers can capitalize on seemingly obscure keywords with little competition.

I want to share how and why I sold the company in order to both educate and inspire potential software entrepreneurs.  I certainly don't know everything and obviously companies sell for much more than I sold Long Tail Pro for (I won't be featured in Tech Crunch anytime soon); however, I'm willing to share what I have learned and hopefully that can be beneficial to a few of you.