I have tried several quite famous forum systems. I found most of them are category-centered style. If user wants to post a topic, first he has to find a category. I wonder if there're some forum systems which are user-centered. That is to say, all topics are structured surround the users. User can post a topic, then he can 'tag' with some categories. I found Flickr do in this way and wonder if there're some such forums.

I'll be very appreciated that someone can give me some hints. Thanks