I can't recommend Akka's state machines enough. It's an incredibly simple and rock solid state machines built right into the library. (http://doc.akka.io/docs/akka/snapshot/scala/fsm.html) We're already on a Play/Scala backend so adding these was incredibly easy for us, but I think they're one of the hidden amazing features of the akka/scala world.
Hmm. I've tried using them multiple times, but have ended up ditching them for one reason or another. There always seems to be one thing that makes it easier to just implement one via a regular akka actor or a regular scala class.
Akka FSM in and of itself does not address distributed FSM's. However, supporting such would likely involve their clustering[0] support and perhaps distributed data[1].
