Many of us who build websites get in the habit of using the Overture "Keyword Suggestion Tool" to do keyword research. It's convenient, and one of the few remaining such tools that is free. Be careful, though, because there are four problems you may run into with it.
1. Singular and plural forms are lumped together. I once optimized a site for "mountain hiking" based on the search traffic indicated by Overture. I later discovered that over half of that traffic was actually for "mountains hiking." Unfortunately, the search engines don't lump singulars and plurals, so those searchers didn't find their way to my site.
2. Alphabetizing. I hesitate to mention this, because it only seems to happen occasionally, but that makes it worse. Sometimes Overture alphabetizes search phrases. If you see "bag lightweight sleeping," you might guess that the searches are actually for "lightweight sleeping bag," but what about "hiking mountains?" That could reasonably be searched for both ways. Look down the list to see if other phrases are obviously alphabetized.
3. Misspellings are not always seperated. I searched "perseverence," on Overture's research tool, an honest spelling error on my part, and saw good demand. I almost optimized a page for it before taking a second look and seeing that the results shown were for "perseverance." You can be certain that searches are being done both ways, but how many each way?
4. Inflated figures. Overture often shows more monthly searches for a keyword than other research tools show for ALL search engines. Who is in error? Hard to say for sure, but given Overtures other imperfections...
Overture's Keyword Suggestion Tool is a fast way to learn if there is some demand for a keyword, and it's still free. For serious keyword research, though, it's best to look to other tools.