I like the Google Keyword tool. Just do a Google search for it & it comes right up. You can see how much it's searched for on average, by locality, the past month, etc. Pretty handy!
Well Mr. SEO Expert UK....I suggest Google Analytics! It will tell you the traffic for each keyword and more. But as an expert you should have already known this?
I recommend you to use Google analytics. before that you have to learn about advanced segments and custom reports. If your using PPC link adword account and analytics accounts otherwise analytic will give mix results.
Thanks for sharing your ideas. I am looking some other tool to know the traffic on particular keyword.