If you're not satisfied with the search box in Safari and want something more powerful than Inquisitor is what you're looking for.
Similar to Spotlight, Apple's excellent search technology, Inquisitor shows results as you type.
Use your arrow buttons or your mouse to then select the result you want. It can be customized, allowing you to choose what search engine to use and where your results come from, how to display results and will remember what you've been searching for.
Inquisitor is very stable and shows results fairly quickly. You'll have to put up with affiliate links in your search results though, but that's the price to pay for a free application.
Also make sure that both Inquisitor and Safari are updated for it to work properly.
All in all, if there's one plugin you should add to Safari it should be this one.
It's a shame that Inquisitor is limited to Safari as I'm sure users of other browsers would enjoy it. If you're hooked on Spotlight, you'll love Inquisitor.