Keyword Research:

These tools aid the selection and implementation of keywords for your site, which will assist you in finding your desired market. Having the right keywords for your market allows you to tailor each page on your site to best suit what you customers are looking for.

Good Keywords Software - http://www.goodkeywords.com/
Good Keywords is a free Windows software for finding keywords for your web pages. The software is easy to use and checks Overture to see how many searches per month each keyword term receives.

Spannerworks Keyword density analyser - http://www.spannerworks.com/seotoolkit/seo-toolkit/keyword-density-analyser/
This tool shows the words on your page listed by number of occurrences - the words which appear most on the page should be the keywords for the page. This tool is ideal for showing which keywords the content doesn't cover.

Thumbshots.com Ranking - http://ranking.thumbshots.com
This site gives you a really clear visualisation of the differences between different search terms. The sites which appear for each search are compared with those in either another search engine or the sites appearing for a different keyword.