Google have released a tool that lets you investigate and compare correlations of search volumes across different terms over time. This lets you see which search terms are always popular at the same time as one another. The tool is called Google Correlate and can be used for PPC purposes.
Google developed this as a result of the flu outbreak a couple of years ago. They realized that simple flu-related searches (“flu symptoms”, “feverish child”, etc.) were very closely correlated with actual flu cases. This information meant that Google could provide easily accessible data about where flu cases were (probably) worst to help planners deliver medications, etc.
But the system they used to set this up had the potential to be more flexible. In addition it can take non-search patterns and perform data-mining to look for the best correlated searches.