WeFollow.com has a fairly simple function: See the top Twitter-users based on category/tags. Created by Kevin Rose, the famous founder of Digg.com, I have little doubt that the site will quickly become filled (as it is quickly doing).
I have added myself (@human3rror) to the list and apparently I’m ranked #1 in the fields I’ve chosen: (#church, #ministry, #christian), which is sad because I’m certainly not the most famous or grossly-followed twitter-user out there that falls in those categories.
So, start populating the list, for whatever reason you might have.
On a personal sidenote, I think directories of this nature provide little value, unless there is energy to actually do something with it. I’m looking into something just like that… stay tuned.