Most of us who develop websites enter the habit of using the Overture "Keyword Suggestion Tool" to do keyword study. It's easy, and one of the few remaining such instruments that is free. Be cautious [http://www.jtfoxx.com/about jt foxx], though, since there are four problems you might come across with it.
1. Plural and singular forms are lumped together. I once optimized a website for "mountain hiking" based on the search traffic mentioned by Overture. I later found that over half that traffic was really for "mountains hiking." Regrettably, the various search engines do not lump singulars and plurals, so those hunters didn't find their way to my site.
2. Alphabetizing. I hesitate to mention this, because it just generally seems to happen sometimes, but which makes it worse. Sometimes Overture alphabetizes keyword phrases. If you see "bag light sleeping," you might reckon that the searches are now actually for "lightweight sleeping bag," but think about "hiking mountains?" That may reasonably be looked for both ways. Look down the list to see if other phrases are clearly alphabetized.
3. Misspellings are not always separated. I searched "perseverence," on Overture's research tool, an honest spelling mistake on my part, and saw good desire. I nearly improved a typical page for it before having a 2nd look and seeing that the outcome shown were for "perseverance." You may be sure that searches are now being performed both ways, but just how many each way?
4. Inflated numbers. More monthly searches are often shown by overture for a than other research resources show for ALL search-engines. Who's in error? Difficult to say for certain, but given Overtures other imperfections...
Overture's Keyword Suggestion Tool is a quick way to learn if there's some need for a keyword, and it is still free. For significant keyword research, though, it's better to look to other methods.