Site Review: Yahoo.com - Forms Evaluation
Yahoo uses Ajax to suggest search terms to users when they are entering a search phrase into the search text box. Once the user has entered a search term and then presses the search button, the browser will post the search form to the search results page. I think that Yahoo is making great use of Ajax in this situation because they are helping users find information as well as suggesting alternative search terms for them to try based on what has already been added.