Locale is my killer app. I literally bought my G1 because I knew that it had this app, and it was the first thing I found and downloaded when I got home that night.
(What's up with the bright idea of "need to view Android Market on a handset" to see a list, in the main page http://www.android.com/market/ ? Perhaps also install iTunes, eh ;-)
P.S. Sorry, the first link is NOT the market page, and it's dated 2008. Can't easily find current price ...
Looks nice, but not a killer app in the sense that the parent is talking about. A more sophisticated settings/preferences is something only a small number of people would use.
http://www.twofortyfouram.com/