iTunes' search powers are functional, but not quite fast or convenient to get to. The App Store Instant web app pulls up App Store results and purchase links as you type each letter for really quick browsing and buying.
The site's own engine only stores the price, icon and first few lines of each app's iTunes store entry, and clicking any link brings you to the app's official iTunes landing page. That's pretty handy for anyone without iTunes installed or those looking for a very specific app without being quite sure of the wording they need. It's a free service and doesn't require a login for searching.
App Store Instant [App of the Day]