I recently asked this question and got back a great solution using jquery for autocomplete:
Need a good way for user to select "To" for email sending
FYI, This "autocomplete" plugin is no longer in development, per a note on the webiste:
http://bassistance.de/jquery-plugins/jquery-plugin-autocomplete/
jQuery plugin: Autocomplete
Note (2010-06-23): This plugin is deprecated and not developed anymore. Its successor is part of jQuery UI, and this migration guide explains how to get from this plugin to the new one. This page will remain as it is for reference, but won’t be updated anymore.
So if you want, you can use the following comparable example from the jQueryUI autocomplete:
http://jqueryui.com/demos/autocomplete/#multiple-remote
I personally prefer jQuery Tools to jQueryUI / but they don't offer an autocomplete plugin at this point... oh well.