Use the Google Adwords Keyword Planner to find new and related keywords, but ignore the search volume data! The search volume data in the planner is really only useful for keywords that you’re actually spending money to advertise on. Otherwise, these volumes are not reliable. While not really helpful to decide which keyword is most used by your potential audience, Google Adwords Keyword Planner makes a useful tool in coming up with ideas for potential keywords!

This is one of the best articles on Keyword research tool that I have encountered. However, To add more to this, I just wanna point out a tool (LSI Graph) for an outstanding long tail & semantic keyword research. After the recent Google algorithm, Google is now, however focussing more on context & semantics than keyword alone. What do you think about it?

Keywords are supplied with accurate, clickstream-based metrics, some of which are exclusive to Ahrefs. For example, Clicks and Clicks Per Search can give you a better idea of how much traffic your content can get. That’s a burning issue today – many queries never translate into clicks because Google reveals answers for them at the top of SERP (i.e., Knowledge Card).
What are you waiting for? Today is the first day of the rest of your internet marketing life. Yesterday would have been a better day to get Long Tail Pro. Tomorrow might be okay too. But today is even better. I’m telling you – when I first got my copy of Long Tail Pro – I was like a kid at Christmas time. It was SO much fun opening up new possibilities that I didn’t have without Long Tail Pro.
As you might imagine, the ability to completely see and study how the competition is ranking for certain keyword phrases in your niche has significant value for any company. Tack this on with its other features, and Long Tail Pro makes a ton of sense for content marketers. And like I said before, the price tag is very low compared to some of the tools I’ve seen out there, with a one-time fee (NOT a monthly fee) of $97 and a 60-day, money-back guarantee.
The limit on manual keywords could be higher.  I personally wish we could input 10,000 keywords at a time, instead of 200.  However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input.  Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).