If you are looking for keywords in languages other than English, you will find Keyword Tool's features very useful. Keyword Tool allows you to pull keywords from 192 Google domains and use 83 Google language interfaces to generate keyword suggestions. That way we make sure that the generated keywords will be relevant to the country and/or language that you are creating your content for.
This is another free tool for keyword research. The tool allows you to combine your keywords in a very easy manner. It has three main match types which include broad match, phrase match and the exact match. You can choose on any of them depending on your keyword search objectives. In addition, you can either use broad modifier or optional keyword or both. Most users are finding it to be a very useful tool in their search for the most appropriate keywords.
This is a rather crude metric because it presumes one can monetize all the traffic they receive AND one can generate as much profit per visitor as Google does. Anyone who could do both of those would likely displace Google as the first consumer destination in their market (like how many people in the United States start ecommerce searches on Amazon.com rather than Google.com).
1. AdWords Keyword Planner - It's still the standard, although Google keeps making changes that just aren't helpful. I get that they want us to treat closely-related keywords in such a way that we're not creating multiple pages when we should just have one, but I'd appreciate it if they'd still break down the volume for each keyword that makes up a group (or at least list the keywords they're clumping together into a group).
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).