LocalTweeps follows a simple premise. Try to gather a list of tweeple by zip code and/or city. You enter your own zip code first and it it launches you into Twitter (via the web interface) to make a tweet about listing yourself.
Inside the site you can browse by city and then narrow it down to the users under that city. A quick way to find local users or target a particular city. I think the UI has some way to go (yes it is beta) and you can pull city info now without having to make users hit another site. But if they add in the ability to give me users in a radius of a zip code also, based on a Google Maps mashup maybe, that would help quite a bit.