The Google keyword tool has the most accurate data and can even give you average cost per click of a keyword when it is used within Google Adwords PPC platform. This can help you identify which keywords are valuable ;)
The Adwords tool can be found here without needing to sign into Google Adwords itself.
There are many tools out there. BUT from my experience google adword keyword tool is one of the approx figure tool. No one can give exact search volume but google keyword tool is too close on 95% of the time.
And guess what its FREE
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