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.
Using the good ol' free Google Keyword Planner is great. But there is an alternate universe of Long Tail keyword phrases that live in Google's other database--namely, their "Autocomplete database." That's the database that is accessed when you're typing queries into the Google search box (notice that list of suggested keywords that pop up as you type?)
I will use the tool to pull in a lot of keywords related to a theme and group them into relevant topics. These topics will either become their own content page or will be combined with other topics to create a page. KeywordTool.io is similar to other tools out there such as Uber Suggest, which I've used for a long time, but it tends to produce more keywords and it provides search volume for the keywords.
The old pricing is never coming back, just to be clear. However, if you bought the one time (lifetime) license at $97, you still have lifetime access to the original version of the software you bought. However, if you want the new/upgraded features (like KC scores), you need to buy into a new plan. If you still have questions, contact me or Long tail pro support at longtailpro com / support
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input. In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).