Google Desktop is a desktop search application that gives you easy access to information on your Mac and from the web. Desktop makes searching your own email, files, music, photos, and more as easy as searching the web with Google. If you're not a Quicksilver user, you should check it out.
It features a Quick Search Box that is your fastest way to do web and desktop searches and launch applications. You can call it up by pressing the ⌘ key twice, and hide it by pressing ⌘ twice again. Type a few letters or words into the search box and your top results pop up instantly, including applications. For example, you can launch iTunes simply by typing "itu" into the Quick Search Box and pressing Return when iTunes appears as the first result.
Check out the Google blog for more information.