TwitterLocal allows you to enter a city, state, and/or postal code and find people Twittering within 1, 5, 10, or 20 miles from you.
A few neat things I've noticed while giving this service a spin: It works. Might sound strange but a lot of times when you try tools like this, they seldom work as advertised. Secondly, I found out that - within 20 miles of me, are people that are not only using Viddler, but watching SteaPTV.
I could see this coming in mighty handy.