Twitter introduced Lists — called TwitterLists – and they are already being used for different applications.
These lists are proving that you give a simple tool to people and they use lots of creativity and come up with all kind of interesting applications.
One of the most novel application that I read about is the list for countries. They want to create list for countries and have already started the process. That means in one list you could address all people on Twitter from a country. Interesting idea — a sure broadcast mechanism to cover all the people. Once you have such lists, you could put it for different uses, including for things like reverse 911 — say to inform everybody about an emergency situation.