This keyword tool was built on a custom database we have compiled over the past four years. We researched data from the (now defunct) Google Search-Based Keyword Tool and also looked at a few more recent data snapshots to refresh the database and enhance our keyword coverage. Our database contains 28,527,279 keywords representing 13,762,942,253 monthly searches. Our database is primarily composed of English language keywords.
Note: If you can’t justify using Ahrefs on a monthly basis, you could sign up for a month, do your KW research and cancel. That said, if you can justify the monthly pricing it’s well worth keeping because you’ll get access to the ongoing functionality such as rank tracking and web monitoring. It also means there’s no need to use any other tools to track rankings or monitor mentions on the web.
You can also filter by query, which is useful when looking at branded queries, or when looking at specific words. For example, only show keywords that include the term "SEO". The graph also allows you to spot trends in across the available metrics and compare week-on-week or month-on-month. This can help you to drill down and monitor progression over time, allowing you to answer questions like "have my branded keywords received more clicks in the last month compared to the previous month?", "has the CTR improved?", "did average positions in Google improve?".
3. Ninja Outreach: Full disclosure this is my own tool, and it is actually an outreach tool, so you may be wondering how it plays into Keyword Research. The fact is there are quite a few data points that NinjaOutreach gets for me that I find useful in keyword research, such as the articles that are ranking for the keyword in Google, their domain authority, their page authority, the number of backlinks they have, and other social and contact data. It's pretty valuable stuff, especially if there is going to be an outreach campaign tied into the keyword research. I wrote a great article with Jake from LTP showing the combination of the two tools.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website.  In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC).  I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.