This is one of the best articles on Keyword research tool that I have encountered. However, To add more to this, I just wanna point out a tool (LSI Graph) for an outstanding long tail & semantic keyword research. After the recent Google algorithm, Google is now, however focussing more on context & semantics than keyword alone. What do you think about it?

After the Panda update rolled out, the latent risk in such a strategy could (typically would) vastly exceed the direct cost of the content, as poor pages on one part of a site could drag down the ranking of other pages on the site which targeted different keywords. Some sites have seen their search traffic fall over 90% with their ad revenues falling even faster. Demand Media went from being worth a couple billion to tens of millions of Dollars. sold on Flippa for $80,000, but was making over $500,000 PER MONTH in profit before getting hit by Panda. Many other Panda-torched sites like Suite101 have simply went offline.
Long Tail Pro by Spencer Haws is the most useful keyword research tool that you can use for your internet marketing. Thousands of webmasters depend on the tool to generate thousands of keywords. It is such effective that it has dominated the keyword search market for the few years it has been into existence. It would cost you less than one hundred dollars to have access to this important life-changing tool.
Keyword Researcher is an easy-to-use Keyword Discover Tool. Once activated, it emulates a human using Google Autocomplete, and repeatedly types thousands of queries into Google. Each time a partial phrase is entered, Google tries to predict what it thinks the whole phrase might be. We simply save this prediction. And, as it turns out, when you do this for every letter of the alphabet (A-Z), then you're left with hundreds of great Long Tail keyword phrases.
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).