So far this is my favourite little UI hack that I've found within Google Chrome. Kind of appropriate that a search engine company would get this right, I suppose.
If you're familiar with Firefox's search feature, Ctrl-F to enter a search term finds the results within the page progressively as you type in the little search box. Chrome moves the search box to the top-right and highlights the answers as you type. Even cooler, it highlights the location of matches in the current page along the side inside the scroll bar, so you can see the frequency of the search term.
That's so obvious now you wonder why nobody thought of it before, particularly given this is the same presentation used by graphical diff tools that developers use every day. Very cool.
Update: Julien Goodwin points out that this feature has been in Opera for over a year. I don't use Opera though, so I've never seen it.