UsabilityDesignPatterns/IncrementalSearch
From MozillaWiki
IncrementalSearch
Much better than types of search that involve typing into a ModalDialogBox or waiting for a progress bar. In IncrementalSearch, you narrow down the results with each keystroke. This is good because it provides InstantFeedback and lets the user rapidly refine their query, or stop typing once they see what they want.
Case Studies
The search box in iTunes uses this pattern to filter songs as you type, and it works very well.