As of September 2014, YouTube's own keyword tool stopped working. It was moved over to Display Planner's AdWords video keyword suggestions. To use it, you need to have an Google Ads account. Just like the non-existent YouTube keyword tool, it was created to facilitate paid video and ads promotion. Which is why keywords that you will find there might not be closely related to the topic that you want to cover.

Usually, keyword suggestions found in Google Ads will be very general because Google wants you to use keywords with high search volume for your ads. These keywords are more likely to generate a lot of impressions and clicks for promoted videos. It makes more revenue for Google but does not necessarily bring more clicks to your content.

TrendingTags is the best YouTube keyword tool alternative. It uses YouTube autocomplete feature to generate highly relevant long-tail keywords about a particular topic. YouTube autocomplete, just as Google autocomplete, was created to make the user experience better. It helps users find videos faster by recommending related keywords when they start typing in their search query.

TrendingTags helps you get over 750+ long tail keywords from YouTube autocomplete by appending and prepending the keyword that you specify with various letters and numbers. To make keyword suggestions more relevant, Keyword Tool allows you to localize results to all countries and languages supported by YouTube.