I've been interested in Modal logic recently. I've been wanting to write a natural language interpreter for a while that would determine certain aspects of a user based on their posting habits. The interpreter would take a user as input for a social networking site like reddit or digg, and it'll parse the comments and posts to determine certain personality characteristics about the user based on their typing pattern. Any previously determined characteristics will be given a percentage rating, and the interpreter will adjust the algorithm based on the more likely outcome.
Initially, the algorithm would be developed to log things like their gender, age, sexual preference, religious preference and any other hobbies they may have. But with time, it could be tailored to look for certain aspects to track down people within a certain group.