I've seen many different optical slaves for flash units, but I am not sure which one to trust. After reading some reviews regarding cheap and expensive units, I am more confused that decided as they often are either negative or mixed.
For example, this Seagull one
is very cheap but is accused of not working most of the time.
I have Pocket Wizards and they are my main workhorses, but I only have two receivers and can't use more flash units as most handheld flashguns don't have have a slave mode. I've found myself in a collaborative photo session where we needed to control flashguns.
One last thing to note is that the unit should also have a PC input.