The Ajax Technology of Google Suggest fame has been implemented in thousands of sites across the web, thereby increasing the overall popularity of the page. But the most terrific use of it for me has been at CPAN Suggest , which is a mirror of CPAN , the ultimate resource center for PERL programmers. As the user keys in the module name, the autocomplete feature brings a dropdown box with 15 modules starting with the letters typed, along with the author name and version no. Quite a helpful feature, more so if you are not sure about module name and thus can check it out this way.
Hope you find it useful too.