I have not used them as much since finding other alternatives, but the one I have used was ReMask.
It comes in handy when trying to mask out really fine things such as hair or tree limbs and branches, but there are many other things it can be useful for to reduce post time.
I like using the tried and true methods, but in a pinch if I am on a time crunch, I will resort to using a plug-in to help speed things along, but only if the results are decent. otherwise it is just wasting time.
Re-mask is okay, seems a little simplistic in the interface, but when you zoom into photos extensively, it renders pretty good results. In a sense, all it is doing is a streamlined process of re-define mask in PS.