Geomeme is a very busy interface, but does a great job of identifying (you are asked first) your location and showing tweets around you. From there you can view hot hashtags or search for your own. It is all wrapped around a Google map mash showing the location of the people sending the tweets themselves.
Rotating tag clouds take up the bottom left and right corners, which is nice and spinny, but quite the eye sore. You can go full screen with F11 and even slow the speed of the updating.
A cool service to find local users and what they are talking about. It also identifies where they say they are located, so you can zoom in and out.