Google Insights for Search is a cute little tool which shows you the relative popularity of different search terms.
The attached image shows how "youtube" came out of nowhere and surpassed "google" in less than 18 months.
The numbers on the graph aren't "real", they are useful only to compare different search terms.
The tool lets you filter your search by geography, by time, or by search category.
If you scroll down to the bottom of the screen, you will see that the tool also show the top ten search term related to your keyword. It also shows the "rising" searches related to your keyword -- terms that you might want to optimize for before your competition does.