3. Microsoft adCenter Labs

Microsoft adCenter Labs has a bunch of neat tools that can be used to gain insight into the way people search. One of my favorites is the search funnel tool, which allows users to see terms that were searched before or after a particular query.
For the term "sofa," the tool indicated that some of the queries that came after were "furniture," "living room furniture," "lazyboy," "couch" and "eBay." The tool gives you ideas for other terms to include in campaigns for more keyword breadth.