Twitter Follower is an unofficial tool that helps you to mass follow users on Twitter.
You just need to browse any page on Twitter with user profiles, click the plugin button and... that's all! You will automatically follow them all.
Here you can watch a video with a demo of Twitter Follower:
Because they will receive a notification about it and some of them (some people say 40%) will follow you back. If you don't want to follow them any more, you can also mass unfollow tons of users with just one click.