Go to http://www.seomoz.org/term-extractor and enter the URL for which you want the keywords/key phrases extracted. Chose 5-6 terms you consider to be most appropriate with the theme of the site.

No automated tool or site can help you until and unless there is human mind working on it.

Identify the keywords using your mind as which keyword suits your business the most.

Also do a keyword ranking analysis on MSN, Yahoo and Google.

Final comments: Keywords are categorized on 1. Sales 2. Branding.

If a site is new use sales keyword which are easy to optimize for and then probably move towards Branding keywords.

Rakesh Tulsiani
aka
rakeshmosaic