Google's AdWords Keyword Suggestion "keyword sandbox." The keyword sandbox was written with Google's AdWords in mind. In addition to broadening the root keyword entered, it also provides related term information (essentially, synonyms). Synonyms displayed may result in an ad being shown for an extended broad match.

Note any keywords or keyword phrases in this area that might require a negative match. Extended keyword recommendations might be killer keywords that belong in your campaign with their own AdGroups listings, plus attendant creative and landing pages.

Online thesaurus. Try this one (or an old-fashioned, offline one). Nothing beats a thesaurus for finding synonyms. That's what they're designed to do.

Online dictionary. I like this one (or try the non-virtual kind). Dictionary definitions provide creative fodder and can stimulate a new train of thought.

Teoma's and AltaVista's search refinements. Several search engines display related searches or search recommendations when a search is performed, particularly on a keyword where the searcher's intent is unclear or ambiguous. These search recommendations often contain related phrases and synonyms.

Recommendation engines may have educated one- and two-word searchers to lengthen their queries for better results. These engine-based tools are particularly good at finding related concepts that take you down a different keyword research path.

KwMap.com. The site's keyword relationship data seems to mirror what's found elsewhere, but I like the unique method of displaying "a network of keywords and concepts as they logically relate to each other, on various topics." The site seems to work on a site directory hierarchy that ties sites to keywords -- as long as they link back to KwMap. I just learned about this site and have been unable to contact the owner to learn more.

Your own campaign is an ongoing keyword research tool when you run broad match listings. The exact phrase used to find your broad-match listing can be captured by a Web analytics or campaign management system, and show up in your log.

Some SEM and search engine optimization (SEO) agencies have keyword research tools available only to their clients. At my company, we used a proprietary keyword research tool internally, then released it to the SEM and SEO community free of charge.

Next in the series, paid keyword research tools. How do you use them? And why fork over your hard-won marketing budget to invest in advanced keyword research (including competitive intelligence)?