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The Free Keyword Tool from WordStream
I myself use Googles Adwords Keyword Tool as it's free and it shows Google data which is important as it's the most popular search engine.
tmongy (27 September, 2009)
Market samurai does all my keyword reseach
Worth every penny
Keyword Elite and Wordtracker and Google Keyword Tools these all are the best for SEO keywords. Keyword Elite just gives more.
Well, I'm currently using only Market Samurai and loved it!
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If you're finding an excellent keyword tool, I'l recommend "Market Samurai". It's easy to use, fast and reliable result. This keyword is all-in-one SEO tool. It has rank tracker too and other useful features. I'm not endorsing market samurai. I'm just making a statement according to my experience..
For keyword analysis I use Google adwords.I like that because of its free usage and few more points which I have listed below..
Google Analytics Graph Comparison
Drive targeted traffic to your site
Google Keyword Too
AdWords Geographical Targeting with Google Maps
Track your results, tailor your ads
Well the best keyword tool to select keywords along search volume and competition is Google Adword tool.I mostly used this tool for searching and selecting the appropriate keywords according to my niche.