Twitter apps: TweetLater

Want to automate your Twitter followers growth?
I am looking for the killer Twitter app that will allow me to continually grow my followers, but without having to spend hours clicking 'follow'.

My first find is TweetLater (www.tweetlater.com). It comes in a free version and a pro version.
By logging in, I can set up accounts to 'manage' my Twitter.

The service will:
- automatically send DMs (direct messages) when someone starts to follow me, like 'thanks for the follow'
- automatically follow anyone that follows me

This has already borne fruits, showing an exponential growth, nearly doubling my followers each day.