Google for some unexplained reason has a tool that shows you how to find keywords to create a cloud of related words that will help rank.
The architecture of a site is best when many semi-related words are all written about all leading to the central star keyword of the site. How to find the words that Google believes are related becomes easier, Google gives us tools.
One is called Google Sets Labs.
If you was going to create a Tag Cloud, this would help you to find all the tags in your cloud...