To find the most popular keywords for bidding on ads or increasing your site's rank in search results (SEO), try the following:

• http://www.serpanalytics.com/tools/top_keywords: Shows keywords by CPC, number of searches, and click rank.
• Use Google Suggest which shows popular keywords as you type. Or try the Google AdWords Keyword Tool which shows popular keywords based on a site or given keyword.
• seomoz.org offers a backlink analysis tool http://www.opensiteexplorer.org/ based on a website. This is a good way to understand the keywords for competitors and yourself. You can also look at your backlinks by using Google Webmaster, Yahoo Site Explorer, and Bing Webmaster.