For some time, we ran a popular series on TGR called "Trends" -- you can see 'em all right here. We used Google Trends and Google Insight to uncover interesting behavioral relationships. Now Google has gone and stolen our thunder, releasing Google Correlate to the world.
Google Correlate lets you directly compare the search histories of various search terms, but it doesn't stop there -- users can also use time series (either uploading their own or selecting from pre-loaded ones) or use various datasets like the latitude of an American state, or whether or not it is in New England.
Most fun (but not so practical) is a feature that lets you draw a curve and see what search pattern matches it most closely -- try it here.
Google Correlate sprang from the same Trends-based tool that spawned Google's flu warning device. In fact, you can see that research in the new tool.