Few sites these days are using AJAX Auto Complete to help users filter known choices or reduce typing. As more AJAX libraries are released this practice should appear more frequently. Autocomplete generally comes in two flavors; Local Auto Complete and remote or server based auto complete. Local auto complete loads all the data locally into memory and then parses/pages through the data entirely in JavaScript. Server based auto complete like Google Suggest send async queries to a server as the user types. Local autocomplete works well when the entire data set is small enough to process at once think hundred’s or thousand’s or items. Ten’s of thousands may be possible depending on how complex the data and what type of matching is used. Prefix matching requires less processing that a full search for prefix, sufix, and contains matches. The Yahoo UI Team has posted a design pattern on Auto Complete and plan to release code as part of their UI Library. script.aculo.us includes support for both Local and Server autocomplete. They have a few demos posted that you can try yourself.
UPDATE: A couple demo’s of my own:
YubNub Suggest
Dojo Autocomplete
