Try SEODigger

[punchy]

With online free seo tool Seodigger.com (http://www.seodigger.com) you can find out for which keywords your site ranks high enough to be in Google Top 20. You can analyze your own sites as well as sites of your competition, using the information obtained for SEO analysis, building a semantic core, finding out how well is a site optimized for this or that search request, and a lot more. Apart from a list of search queries, wordtracker and overture results are also determined (keyword popularity calculated by Wordtracker.com and Overture.com).

[Trusttec]

I did a quick search but it appears you have to register to get full access?

[Jean Costa]

works fine for me. Very useful website :)

[IFOR2005]

that was nice.. thanks for sharing.. i stumbled on this before. but i did not try on how it works...