Inquisitor is a great plugin for Safari that enhances the web search field. As you type, Inquisitor offers search suggestions. For example, typing ?ban? will return ?bank of america?, ?banana republic?, and ?bandwidth test?. These suggestions come from your personal search history and from the search history of all Google users, and it's done in an animated and slick way.
This is already a great plugin, and it should get even better in future versions.
Update: It did indeed get better with future version, but it has also left the wonderful world of freeware. There is now a mandatory donation.
Updatex2: It is now freeware again. Yeah Freeware.