Self-similar groups acting essentially freely on the boundary of the   binary rooted tree
  We study the class of groups generated by automata that act essentially freely on the boundary of a rooted tree. In the process we establish and discuss some general tools for determining if a group belongs to this class, and explore the connections of this class to the classes of just-infinite and scale-invariant groups. Our main application is a complete classification of groups generated by 3-state automata over 2-letter alphabet that are in this class. 
Rostislav Grigorchuk and Dmytro Savchuk
math.GR