1. Google Keyword Planner - When you input one keyword, multiple keywords, or even your website address into Keyword Planner, Google will give you a list of related keywords along with simple metrics to gauge how fierce the competition is around each one and how many searches it gets on both a global and local search level. It'll also show you historical statistics and information on how a list of keywords might perform -- and it'll create a new keyword list by multiplying several lists of keywords together.
2. Google Trends - Google Trends is a free tool from Google. It lets you enter multiple keywords and filter by location, search history, and category. Once you enter that information in, it'll give you results that show how much web interest there is on a particular keyword, what caused the interest (e.g., press coverage), and where the traffic is coming from. It will also give you similar keywords. If you're trying to decide between two keyword variations for your latest blog post title, simply perform a quick comparison search in Google Trends to see which one is getting searched more often.
3. Keyword Tool.io - A long-tail keyword is a keyword phrase that contains at least three words (or two). They are used to target niche demographics rather than mass audiences. Say, you're a death metal band specializing in songs about hell. Well, that would be pretty specific and you would need long-tail words. Keyword Tool gives you a list of long-tail keyword suggestions related to one you already have in mind.
What Keyword Tool does is use Google Autocomplete to generate a list of relevant long-tail keywords suggestions. The search terms suggested by Google Autocomplete are based on a few different factors, like how often users were searching for a particular term in the past.
4. Moz's Keyword Difficulty Tool - This program cost money but there is a free 30-day trial which can help you develop your website free. When you input a keyword into this tool, it'll find the top 10 rankings for that keyword and a "Difficulty Score" based on the pages that are currently ranking. You can look at search volume data (how many people are searching) for your keywords, then pull up the SERP to see the top 10 results for each term.