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Google's Zeitgeist [google.com] gives ideas for trends and very popular keywords. The Google Adwords Keywords Suggestions [adwords.google.com] tool is probably more useful to you.
It's probably not what you wanted to hear but i think you are to late if the pages are allready built. You should check words and phrases before you start to build or optimize a site. Then you'll know which are the most important words.
I prefer to run test campaigns with the real keywords and phrases i allready optimized the pages for. These words ARE relevant and by running a test i get a pretty good picture about how many searches are done per day and how good their performance is. I also learn which pages/topics are worth to expand or to rework. Better than with any suggestion tool.