Yesterday afternoon, my coworker Kelly was sharing with a few of us at Cirrus ABS the results of a Twitter experiment that he has been conducting since August. While I have been sworn to secrecy about what we talked about, he did point out a few websites that I forgot about.
FollowerWonk.com is one that gives you some interesting stats about you and others.
FriendorFollow.com is a tool I use on a regular basis to clean up my 3000 or so Tweeps that I follow.
One more tip, if you have discovered that someone you are following is not following you back, perhaps they unfollowed you by accident, or Twitter itself has been known to drop followers.
If you notice that someone you follow is not following you, simply unfollow them and then follow them again and they will get a notice that they have a new follower (you) and they may start following you.