Go here to search all tweets on a particular subject. I just put in Seattle and found a bunch of stuff. So, how would/could you use this??
Lastly, here’s the app I believe Kathy was using. Note, you can “group” people you follow/tweets according to subject.
I am using something called Twitterfic, which is what David Pogue mentions in his column. (Go back and read it if you haven’t, below). There’s also something called Twirl and I bet there are a zillion others. Explore if you’d like.