Lifehacker reader Marat Levit wrote in to point his script out to us. Once installed (using Greasemonkey), you can alter the site you want to search on by either clicking the Google logo or pressing the Up key. Do either and the logo (and associated search options) change.
Choices offered are Bing, Wikipedia, Twitter and YouTube. If you set Google as your home page (and don’t use the browser search box), it can definitely be useful. Thanks Marat!