With all of today's TTL, Wireless, Air, and Radio control of the flashes, we often overlook simpler things - items that have been developed from older technology.
Suppose you wanted to use several old flash guns for a portrait setup and they were of different makes - perhaps they have no TTL connectors on the bottom at all. Maybe they don't even have hot shoe connections - just a PC synch cord coming out of one side of he flash.
Also suppose you have very little money in your pocket. How are you to synchronise the extra flashes to your camera? With one of these:
This is a simple little electronic cell flash synch. It completes the basic flash circuit when the sensitive cell on the front of it detects a pulse of light. It has a 1/4" threaded socket on the bottom, a hot shoe on the top, and a PC connector on the side - you can cater for all the old flashes you want to.
Fire a basic master flash from your camera - it can be the in-built one or something that you sling on the top. You needn't even let the light from this hit your subject - you can shield it with a piece of alfoil or cardboard. As long as the light of that basic flash reaches the electric cell on the humble slave, it will fire off whatever is connected to it.
Cost? $ 18.95.