Using the right keywords in your web pages is important for SEO. The keywords you include in your pages decide for what searches they will show up.
Diggol offers a free tool http://www.diggol.com/IIMintro.htm that will tells you on each of the three top search engines, what keywords are associated with any keyword you enter.
You can enter a keyword and find a long list of keywords that are associated with your keywords and are contained in Google, Yahoo and MSN search results. Hope this is useful.