The Display Planner tool replaced the YouTube Keyword Tool in 2014, and can be accessed from the Tools section of your Google Ads account. Although it's a little more involved than entering a keyword into the YouTube Keyword Tool, the Display Planner offers some great functionality for making the most of Display campaigns, including keyword-driven video ad campaigns on YouTube. The Display Planner also offers precious insights into the demographics and interests of your audiences, allowing you to create highly tailored Display campaigns.
Google's AdWords Keyword Planner tool is another common starting point for SEO keyword research. It not only suggests keywords and provides estimated search volume, but also predicts the cost of running paid campaigns for these terms. To determine volume for a particular keyword, be sure to set the Match Type to [Exact] and look under Local Monthly Searches. Remember that these represent total searches. Depending on your ranking and click-through rate, the actual number of visitors you achieve for these keywords will usually be much lower.
Although more and more keywords are getting encrypted by Google every day, another smart way to come up with keyword ideas is to figure out which keywords your website is already getting found for. To do this, you'll need website analytics software like Google Analytics or HubSpot's Sources tool. Drill down into your website's traffic sources, and sift through you organic search traffic bucket to identify the keywords people are using to arrive at your site.
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.
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.