If you are a Safari user, you must give Inquisitor X a shot. This is a donation-ware utility that greatly enhances Safari's search box. As you type, it looks up likely search results on sites like wikipedia and Google, and populates a dropdown with the best hits. This is insanely useful. The 3.0 beta adds search history weighting to the results (all stored locally; no spyware).
(Never mind me. I'm just bitter 'cause I paid for it back when it was for money and he supported Camino. Then he yanked Camino support in a sulky fit, then he made it free. Oh well. I have supported an independent developer; have done much more pointless things in my life.)