Tweeter Search and Tweet Scan help to find people who has twittered about something in particular, but what if that person twittered about a subject (like cola drinks) without making specific mention to the subject (the tweet just said: "Ah, I needed a cold Pepsi!")? In this case, the search engines mentioned above will be useless.
Here comes FlockUp, a kind of special interest directory for tweeter users, where you can look up for a subject, event, whatever, and if that "flock" already exists in the database, it will list the people who is interested in that subject, then you can click through to their Twitter profiles and decide if you want to follow them. If that subject has not yet being entered, you can create a flock for it, and then add your Twitter name, so people can see you are interested in this matter.
There only one "big" issue with this site: anyone could add your twitter name to any flock, and there seems to be no direct way to remove yourself from a flock if you consider that appearing as interested in that particular subject is not good for you.