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1). If your site is related to webmaster? e.g. Link popularity, seo, and web hosting.
2). If your site is general web master directory?
3). and Last any of keywords use also by non professionals people.
Waiting for your views!
You can't expect from most webmasters that they will tell you which keywords are mostly used to visit their website, but Google maintains a list of the most used keywords in previous years. It is called Zeitgeist and gives a broad idea of what people in the world were searching for in the past. The 2005 list can be found at [google.com...]
One more tool that might help you - Overture's search term suggestion tool:
Put in a word and you'll get an indication of how often that word is searched for in a month, as well as what phrases it is searched in.
To check the competition of a term, I use the "allinurl" and "allintitle" tags for google, they proved to show pages that are well SEOed and give a clearer picture wheter or not a niche is a niche beyond the pure overture-list.