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.
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).