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?
I also want to emphasize that Microsoft Excel and Google Docs perhaps play an equal, if not more important role in keyword research than any of these tools, because at the end of the day, the mountain of data you end up with is of no use if you can’t perform calculations, sort, filter, present or store it in a way that’s best suits the task at hand. Head over to Distilled for a comprehensive Excel for SEOs guide. Alternatively, bug this guy—he’s the Sensei who trains Excel ninjas in the dark of the night.

The WordStream suite of Keyword Tools is an integrated and actionable set of keyword tools designed to help search marketers with keyword search, keyword suggestion, keyword grouping, PPC keyword research, keyword analysis, long-tail keyword research, and negative keyword discovery, going beyond the capabilities that a typical free keyword tool can offer.
Mentioned in the above video is the Keyword Competitiveness (KC) feature within Long Tail Pro. With earlier versions of Long Tail Pro, you could only access this feature if you paid extra to upgrade to the Platinum version of Long Tail Pro. Pricing structures and the way the software is delivered has evolved over the years – and currently (as of the date I am writing this) all features of the software are included no matter which payment plan you choose. So right now – everybody who buys Long Tail Pro has access to this extremely powerful feature.
Spencer, it seems like you got out at just the right time and made your money? Seeing as Google has basically stopped giving access to their free keyword tool. I own the FULL lifetime license $290+ I think is was. Now Longtail Pro is useless due to the restrictions that Google has put in place. Do you know what the future plans are for Longtail Pro to get around this issue that I’m sure everyone is facing? Or can you get some feedback from the new owners?

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).