SiteVolume is a recently launched service that lets you see how often search terms appear in various websites.
From the makers of TweetVolume, SiteVolume is really an extension of the original service. Going beyond Twitter, you can now see the frequency of your search terms (up to five) for more websites, including Digg, MySpace, YouTube and Flickr. You'll need to first choose the tab for the site you'd like to search. Then enter your search terms. The results will appear in the shape of a bar graph, indicating how many times each of your search terms appears on that site.
Since the team is so dedicated to the ongoing improvement of its services, hopefully they'll be able to add comparison graphs in the near future as well. I'd love to be able to see the trends across the different sites.