I Suggest Google Suggest

Google Suggest guesses, what you are typing and gives you some suggestions in real time. It is something like the 'Did you mean?' option that gives you some alternate spelling suggestions for your query, after your search. In Google Suggest, for example, if you type 'apple' it might give you the list of suggestions that include 'apple ipod' or 'apple mac'. You have the option to choose any one from the list by using up or down arrow keys or using the mouse.
They are using the upcoming technology AJAX(Asynchonous Javascript and XML) in order to implement this feature.