jQuery users, take a look at the autocomplete plugin which is shipping in the UI framework. While there are some serious limitations to the currently plugin, it does provides a drop dead simple way to provide Google Suggest style functionality to your site users without significant overhead. It’s widely supported and weighs in at 78K (without data/css and markup) for the minified components.
My one complaint is the lack of JSON support for remotely fetched results. Currently you must return items for the suggest widget as plain text list. You can, however, currently parse individual lines (which could be objects). I expect the jQuery team will be adding this functionality soon enough. Jörn Zaefferer has built a modified version of the plugin that supports JSON. I’m holding out until it’s merged into the UI trunk to limit my sources of code updates in this instance.
Below is a quick example of the syntax so you can see how easy it is to create. The file, names.php, simply returns a series of names, 1 per line. This might be static or pulled from a database based on the passed “q” get parameter that holds the contents of the input area.