Setup Google Analytics on your website, and as others have said, use the Google Keyword search tool. It gives you a good idea of A) what the hot search items are for your site and B) solid keywords that may not be as competitive on PPC pricing.
One strategy I've been pursuing is passing on the most searched keywords in my PPC campaign, but focusing on smaller market keywords where the cost per customer acquisition is miniscule. This works especially well with location based services (I explicitly specify cities where my services are offered). Not only is this cheaper on a CPC basis, but it also is a much higher quality lead than someone doing a general search.