The phottix strato II system also has TTL pass through. You put the transmitter in your hot shoe, and it has its own hot shoe on top. You can mount your TTL controlled flash there (or use an TTL cable there to use the flash in a different location). Put a Strato II receiver on your other flashes, and they will fire in manual mode when you hit the shutter.
The strato II system is like $99 for a transmitter and reciever, plus about $60 for each other receiver. I have found mine very reliable.
Some flashes have an optical sensor built in that can fire them when your TTL flashes go off. However, make sure you look for a model that has a setting to fire on the second flash, so it does not go off on the metering flash. I know the Lumopro 160s have that feature. Not sure about others b/c the Lumopro is all I've got.