Keywords and keyphrases play the most important role in web sites traffic, and finally -lead generation. We don't like to drive tons of traffic and not get any business from it. That's why promoting your web site you need to find most relative keywords and keyphrases to your content, products and services you sell.
Personally I prefer the following tools:
Digital Point Keyword Suggestion Tool
Overture Search Term Suggestion Tool
Recently I found two very interesting tools provided by Google:
Google AdWords Keyword Tool
Google Sets
Check them out - I really like the Google Adword Suggestion too and will try to use it to promote one of my web sites. Also, here is a list of Keyword Suggestion Tools - may be you will find it useful.