Keyword research is an activity you undertake every now and then. If you have a clear definition about the product or service you want to ‘sell’ with your website, you should be able to come up with keywords, related keywords, and even more related keywords to make your awesome website (more) findable. As your product and the market will evolve, your keyword strategy should do the same.
"I rarely get excited about SEO and SEM tools...But, when it comes to keyword research it is virtually impossible to produce a quality worksheet without software. When I have to perform the same task for my customers, the process is even more daunting and time consuming...there is now a better way to manage huge keyword lists and this tool is called WordStream."
Joost developed his own keyword research tool to come up with keywords as well! Yoast Suggests uses the Google Suggest functionality you know from searching in Google. It finds the keyword expansions Google gives and then requests more of them. So if you type ‘example‘, it’ll also give you the expansions for ‘example a…’ till ‘example z…’ etc. Just go on and try it and fill out some of your potential keywords. It’s a great way to quickly find more long tail keywords you can focus on.

Google Webmaster SEO Starter Guide - Originally written for Google Employees to improve website performance with users and search engines. Related Terms: SERP - Search Engine Rank Position  |  PageRank (PR) - Named for Google Co-founder, Larry Page. The PR Scale of 0-10 rates a web page. In the past, rank was based on the number of high ranking links pointing to the website. Recently factors like relevance and quality have gained importance. This website's current PR rating: 
But it’s still very useful for getting search volume data (provided your account still shows this), which is helpful when choosing which of the many keywords you’ve found to focus on (although you should take these estimates with a pinch of salt, they are still useful in indicating the relative search volumes of different keywords, even if the absolute estimates are a little off).
It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.