Google Insights for Search is quite an interesting tool.  Even though its been around since 2008, it is still in beta as is most of Google.  There is a lot of intelligence that can be gathered by using this tool along your web analytics package of choice.  The only drawback to me is that the search data doesn’t go back farther than 2004.  With this tool you can gauge interest and demand for the search terms you specify and you can compare multiple search terms in the same ‘report’.   There is even a bit of forecasting so can test out Google’s trending and forecasting prowess over time.  Try it out and compare the data you see with that from your web analytics package and other tools such as Quantcast.  Enjoy!