Matt describes the inspiration for the WordPress AJAX plugin on his blog:
I use the Spotlight feature in OS X an awful lot. While some people would rather use things like Quicksilver, I like just hitting command+space and typing in what I’m looking for, be it a document or even an application I want to open.
That’s the inspiration for AJAX.search, a search system built on Prototype’s AJAX framework. It will make a call to a URL you specify and display the results under your search box like Spotlight (and now, like the search at Apple.com). From there you can hit the up or down arrow keys to select a result and then hit the enter key to visit the link.
For more information on the plugin, to see the search in action, or to download it for your WordPress blog, visit Matt King’s blog.