The best way is to follow people that have similar interests to you and Twitter will suggest you to people that follow them, also keep your twitter upto date!
If you will do regularly tweets and it should be effective, then you will automatically gain followers without following them. Don't need to use any online tool.
Yea, imho the easiest, cheapest way is to follow users with same interests. If you want "massive" followers number you may use all these tools the others saying.
Though there are some tool like Twiends, twimates etc to increase the followers but it is better to attract the followers by regular update of your wall with some interesting, informative tweets etc