I recently tested a free chome extension called Twitter Follower. It has free and pay-for versions. It's not bad, but every time I used it Twitter picked up on it.
Free version will perform an action 20x before stopping. Then you can just run it again. Usually on the second time running it, Twitter recognized and put a hold on the account.
It also has a feature to spread out the action over a set period of time, and still twitter was able to pick up on it. Yet, when I do things manually, I can follow/unfollow much more before I hit the limits. I think it might have to do with timing as the bot does things exactly on same intervals.
I question the usefulness of this anyway, as non-engaging followers are useless mostly. As stelios0 said, good content trumps all.