If you want to discover keywords for which a particular site is ranking, analyze it in the Site Explorer tool and click the “Organic keywords” report button. 

To find the best keyword ideas from this list, you can apply filters for various keyword metrics and for a key phrase length. You can even extract keywords, that trigger definite SERP features.

Switch to the Top Pages report to see these keywords grouped by pages. This report will show you pages of a target website ordered by the amount of organic traffic they bring. To see the recent changes in organic keywords, use respective filters.