Follow me on Twitter: @hendricius
I use Market Samurai, Google adword tool and Micro Niche finder, triple check the results against each other and make my decision.
I would not recommend using any paid tools...the best ones are the free.. Google Kw tool shows you the search volume...and you can use any plugin like 'SEO for Firefox' or 'SEO Quake' to determine the strength of top competitors.. I use Traffic Travis as well, and its free..
Using paid tools which gives you a 'green' or 'red' signal is something I won't recommend at all..never rely on a software like MNF or Market Samurai..they are not always right..
Spyfu is something that i would recommend. Basically it can be used to give you a rough idea of the average cpc and performance of a keyword. Yeah, it's web based and Free.
The Secret Definitive Guide To Finding Tons of PR4-PR8 Backlinks with ease <== Boost Your Website Rankings Overnight
I use a combination of wordtracker and google tool.
The combination of google keyword tools and seocockpit do a great job for me.
As per me google adwords is the best tool to find different combinations of the keywords and also to get the best keyword as per the own field. And one can know how much that keyword be found more or less.
And manual research is also the best to know about the best keywords as keyword is the word that enter by people for their search. And one can make observation for such kind of keywords that found more time by people in own field and can use those keywords to get best results, Though this process will take more time to show the results but those will generate far better results.