Zmarties

Google Suggest is certainly serving to popularize XMLHttpRequest and as Joel says "it's going to teach web users to expect highly responsive user interfaces" of which this mapping server is a worthy example (which ironically does not use XMLHttpRequest).

However, one example that does not show the technology to good effect is this Live Google Results which tries to give a "search as you type" effect in Google. The problem is that Google's search does not allow prefix searching, so the searches you get as you type extra characters are unrelated - you are not refining the search with each letter added, but rather getting a completely different set of results each time.