There's no software but you can use a couple web apps and some tricks to help. What I usually do is find someone who tweets similar things as me and has a ton of followers. Once I find that person, I'll see who is following them. I'll add about 100 of the people who follow them and only people who seem to be active on twitter lately. I do this assuming that if they're interested in that person, then there's a good chance they'll be interested in me since I tweet similar things. Then I use a site called ManageFlitter.com about a week after I added those people. On that site, I can see and defollow anyone who hasn't followed me back or anyone who isn't using twitter anymore. Using that process every week...I add about 50 good followers a week and get rid of about 75 who suck.
Ofcourse there are softwares like tweetadder, and some other sites like twiends, twello, wefollow. But the effective way of getting followers is finding the users who has same interests and follow them and interact them with DM in twitter.
There are tools like Tweetadder and Tweetattacks, but they can be time consuming to use. I recommend a service that gets targeted Twitter followers for you - http://www.tweepboost.com/
There are some Twitter web apps that can help you get followers. Like in Twiends, members follow you to get something you offer them. However, you may not get targeted followers using this method. Apps like tweetadder will help you build steadily a follower base of very targeted followers. It may take some few weeks and months but you have a juicy account that can be well monetized.
I have created a similar script. My first client bought a copy and set up http://credfollow.com. You can get followers. However, there are still some little issues as my client is Egyptian where there was instability so we could not finish the setup. I'm selling the script
Actually there are several hundred software sites that can help you get Twitter followers.. Just google "Twitter following software" and you'll find everything you need. Don't believe me, check out my account
I would recommend tweet attacks. You can effectively market on twitter, sometime without the need to have a massive following.