Depending on your topic / vertical and your geographic location the search engines may have vastly different search volumes. The tool can only possibly offer approximations. Exact search volumes are hard to find due to vanity searches, click bots, rank checkers, and other forms of automated traffic. Exceptionally valuable search terms may show far greater volume than they actually have due to various competitive commercial forces inflating search volumes due to automated search traffic.
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.
Joost developed his own keyword research tool to come up with keywords as well! Yoast Suggests uses the Google Suggest functionality you know from searching in Google. It finds the keyword expansions Google gives and then requests more of them. So if you type ‘example‘, it’ll also give you the expansions for ‘example a…’ till ‘example z…’ etc. Just go on and try it and fill out some of your potential keywords. It’s a great way to quickly find more long tail keywords you can focus on.
Well, if you haven’t heard the news yet, the word on the street is Google has shut down their widely popular keyword tool for public use. Now if you want access to Google sanctioned keyword data you have to sign up for an Adwords account. Of course there are a ton of theories surrounding why Google would make this decision, some are more obvious than others. But to many of us Google’s motivations seem pointless when we still have work to do! Luckly we exist in a vibrant industry where innovation is at the core of most of what we do. As such we have a slew of different tools available to replace Google’s tool.

As for the history of the company, as I explained, I created Long Tail Pro back in 2011.  At the time, I was creating tons of niche sites and was doing alot of keyword research and content creation.  I was trying to figure out what types of keywords would rank quickly in the search engines, specifically Google, and I was frustrated with the other keyword research tools (in particular Market Samurai) out there.