I'm wondering if anyone has developed a typeahead jump menu to replace the search field.

Let me explain:
On my site, people do not need to search by individual links, they need to search by categories.
I want it so when a user starts typing the name of a category, a list drops down from the search field and narrows as the user types characters , until finally the user clicks on the appropriate category name and is taken immediately to that category.

Thanks!
George