You should check out IFTTT (stands for "if this then that"), it's more of a generic automation tool but it's pretty incredible once you get your mind wrapped around it, imho. I don't use it for Twitter but i imagine it can do what your looking for.
The only other free one i can think of off the top of my head is friendlybot (google friendlybot should be 1st result) but i'm not familiar with it at all, I've only heard about it through a different community.