The best way to get quality twitter followers is just follow some relevant people and then retweet what they are tweeting.
I have built a whooping network of over 100k followers. I did this by using relevant #HashTags ReTweets and recommendations. When I first started out I did try plenty of free and paid tools however these days Twitter has some pretty cool limits on new and existing accounts so almost anything on the market these days can cause violation problems. To be honest once you pass your 2k mark it will almost run on auto pilot anyway. I do obviously use many Twitter APIs to control certain aspects of the campaign however i do not and nor would i ever use anything to gain me followers. I hope this helps.
Interacting with your followers helps with the quality, I think. They know you don't send out spammy tweets. If you want quality followers it takes a little more patience. Find tweeters in your niche and follow them.
I recently wrote an article on this topic. Check it out here: http://hubpages.com/hub/How-to-Get-Hundreds-of-Twitter-Followers-in-no-Time
A great way is to use "other people's followers" to make you money. You can become an advertiser or you can become a tweeter. An advertiser pays a tweeter to tweet a message, and a tweeter gets paid to tweet someone else's message. SponsoredTweets.com is a great way to go. Hope this was helpful.