Google has a beta search suggest site up. When you type your query into the search field, the field will suggest to you search terms. This is not an easy problem to solve over the web, as the page has to communicate with a server without refreshing the page.

Chris Justus disected the code and explained it.

A working demo and code for how to implement a field that suggests to the user what they want to type has been posted by these guys.

This is cool stuff.