Yahoo! Search made an interesting bit of caching. To see it in action, go to the main search page with Firebug enabled, or any tool that lets you see network traffic. Then type any character into the search input box and you will see some traffic kick off to download items such as:
What are these files? They are artifacts for the results page. So, Yahoo! groks that you obviously are going to do a search once you start to type something in, so why not go ahead and preload the files that are needed as part of the results page?
Search, Yahoo!, Yahoo Search, Contextual, Cache[ /tags]