Mutuality

Well Huitter.com offers a service called Mutuality which has the following features and are very useful in the journey of becoming a Pro-Twittrer.

• Unfollow all who do not follow you back
• Unfollow all
• Follow all who follows you

I dont use the 2nd one but the 1st and 3rd are quite useful. One can also use the 3rd one as a manual auto follow feature.

Twitter Follower

is a database of Twitter users who will follow you if you follow them. By following the users in the database, you can quickly and effectively build your list and avoid that dread zero in the follower count. But everyone doesn’t follow you back in that list that’s the drawback as anyone can submit there name there.
Another feature from Twitter Followers:

Mass Unfollow Everyone Who’s Not Following You: The other feature of Twitter Follow is the mass unfollow button. Push this button and Twitter Follower will unfollow everyone who’s not following you back.

Source: http://www.johnchow.com/twitter-follower-my-new-twitter-site/

See Also http://www.buzzom.com

Other Useful Twitter Resource: http://www.askdavetaylor.com/how_to_auto-follow_twitter_followers.html