Hot Shoe - Cold Shoe - Flash Jack Again
Once upon a time there were glaciers covering the land and only cold shoes on cameras. Flash guns of various types were slotted onto these and the photographers then left to devise their own ways of synchronising the things - and there were any number of ingenious devices bolted onto the tops of Leicas, Niccas, Leotaxes, Contaxes, and Canons to do just that. Sometimes they worked.
Then a genius somewhere decided that spare dangling wires and delicate plug ends were unnecessary - the business of providing two contact points and a positive mate for a flash gun could be done at the base of the flash itself - as you slid the shoe on it made a connection for the electrical circuit. The flash might have been only on-camera but you could be pretty certain it would go off when you needed it.
Then the idea of firing a flash off-camera started up. First it was a length of insulated wire running from camera to flash - it was strong enough to trip you over in the dark and weak enough to break every other time. If you scored the trifecta you tripped, pulled the flash off its stand, and crashed the camera to the floor. Pretty soon people wanted wireless flash triggers - they got them with infra-red or radio signals and a bewildering variety of instructions on how to set them up.
For those of you who have an off-camera flash that needs only a stern command from the camera itself we can go no better than to suggest the Frio shoe. Simple, sturdy, and cheap. More pocketable than the flat base ones.
For those who need to mount a flash and somehow feed a wire to it - remember the trip and crash - there are a number of Promaster models - again simple and cheap.
For the adventurers who would like to slave one flash off another really cheaply there is a Promaster optical slave block.
And if you have gone and lost the accessory shoe that the Nikon or Canon people provided with their flashes - and we know you do lose them from time to time - there is a a Promaster replacement that does exactly the same thing. It is inexpensive enough that you can go and lose it too - we got more.