e3KWD Check is a lightweight, fast and free program for analysing keyword density within web and text documents.
This tool allows you to run real time keyword density analysis tasks upon your local copy while writing or editing. If you have an active internet connection it will also retrieve and analyse online web documents using the built-in address bar.

It is important to note that there are no magic rules for keyword density levels. If you need to ask this question then you should probably not be using this tool. Web documents that have keyword density levels set too high can attract penalties from the major search engines; including being banned entirely from their listings. However from a practical perspective using very high keyword density levels creates unnatural copy which your users will find unreadable. Remember, write your site copy primarily for the benefit of your users not the search engines. Please be sensible in your use of this tool.