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.
You can use Keyword Researcher Pro with the best free keyword research tool out there, Google KW planner. Importing and exporting data is as easy as clicking a button. It’s all done at lightening speed too. There are tutorials of how to do this in video form on the sales page here and they show just how easy this tool is for finding those long tail keywords.
This tool was originally created to carry out keyword research for paid campaigns using Google's AdWords platform. However, it can also be used to research organic keywords, providing estimated global/local monthly search volumes for keywords (i.e. their popularity) and their competition (i.e. how difficult they are to rank for) for organic search.
Keyword Researcher is designed to search the Autocomplete databases of Google, YouTube, and even Amazon.com. That's a lot of keyword sources! As you probably know, if you're building an authority site, or if you would like to become a dominant player in your niche, then starting with a large database of keywords (that your customers are actually typing in) is a powerful marketing tool.
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.