http://friendorfollow.com/
A quick rundown on who of those you’re following are not reciprocating. Sometimes I’ll get a notice of a random person started following me, and if they’re a real person with updates and a website, I usually follow them back. Often those people will unfollow me after a few days (and you aren’t notified of this). So, unless I find them especially interesting or helpful, I stop following them. This tool helps with those types of people.

http://doesfollow.com/
This is a more specific check if a person is following you.

http://twitoria.com
You can use this tool to check for deadbeat followers. If people haven’t updated recently, you can unfollow them. Unless someone is important for me to follow, I unfollow if they haven’t posted in a month.

http://twittercounter.com
You can see how your follower count goes up or down over time.

Feel free to follow me on Twitter.