In yet another in a series of dizzying announcements in the search world, Yahoo! introduced a "contextual search" tool, Y!Q, that allows users to conduct a secondary/search within results by highlighting text on a page. The current offering (in beta, of course) is arguably not set up for widespread consumer adoption. But the concept is right and, directionally, this is the future of relevance.
It's not going to matter how big your index is anymore. It will only matter if you can get me to the results I want quickly and effectively. AOL is trying some similar things with Vivisimo. And FAST is doing some pioneering work in the enterprise space with contextual relevance.
As many have commented, Y!Q's capabilities would enable AdSense competition. Blinxx also has a contextual search capability.
Chris Sherman has a nice write-up of the Y!Q details.