I always use the Google toolbar. This is very easy to use and is known to provide you accurate results. In case, you are completely new in SEO and do not have much of knowledge, it is best to use the Google adwords keyword tool for all your search options.
There are a lot of tools available to check keyword density to help you gauge your site ranking on the search engine. I would like to suggest few of them like:
keyword density prominence tool-If you want to analyze meta keyword or otherwise if you want to focus on one particular word.
keyword density link analyzer-It will help you analyze Alt tags and links also.
Keyword density checker- it has one unique option that is mailing option. It sends the results into your e-mail id.