I received a single LumoPro CAD1026 "Universal Hot Shoe Translator" for Christmas, but it really only works if you have two. When I went to buy another one, they'd been discontinued and replaced by the LP907 Universal Hot Shoe Translator II. So that's what I bought.
"Just what the heck IS a universal hot shoe translator?" you might be asking. Good question.
Simply put, it's the cheapest, most reliable way to fire a flash off-camera.
(The cheapest way is with a low-end optical slave unit, or if your flash has an optical slave built in, as mine do. But those can be unreliable, especially outdoors.)
The LP907 consists of (1) a hot foot with locking wheel on bottom, (2) a hot shoe with spring clip on top, (3) a PC sync plug on one side, and (4) a 1/8" miniphone plug on the other side. There are other "hot shoe adapters" out there, but few of them have all four of these features.
Compared to the CAD1026 (older model), the LP907 is slightly larger (see image below). The PC sync plug has been moved inside, instead of sticking out waiting to be broken off. The locking wheel has a smoother turning motion. It has a clip inside the hot shoe, the better to hold your flash. And it has the company name on it.
How it works: (1) put one in your camera's hot shoe, (2) put one on the hot foot of your flash, (3) connect the two with a PC sync cable or — even better — a cheap audio cable with a miniphone (1/8") jack on each end. That's it. When you press your camera's shutter button, the flash should fire.
In the image below, I'm using an audio cable that came with some computer speakers I bought 14 years ago. The other end is plugged into the CAD1026, which is on my camera's hot shoe. (The tiny screw visible at bottom is part of the LP679 umbrella swivel.)
The unit sitting on my camera's hot shoe has its own hot shoe on top, so I can put another flash up there, if I want to. Both units still have a free plug, and each could therefore fire yet another flash if I feel like having cables draped everywhere.
The upsides to such a setup: (1) extremely reliable, because it doesn't depend on line of sight or an iffy radio signal, (2) extremely low-budget: each LP907 is only $15, (3) you probably already have a 1/8" miniphone audio cable in your house that's not being used (if not, they're cheap too), and (4) does't require batteries — unlike radio triggers.
The downsides: (1) you're limited to the length of the cable, and (2) adds about an inch to the height of your flash — it puts your flash more off-center if shooting through (or into) an umbrella.
Even if you're into radio triggers, which I am, it's not a bad idea to have a couple of these and a long cable in your backpack, just in case.
Note: No TTL information is transmitted to the flash; there's only a single pin in the hot foot and hot shoe of these babies. These are meant for all-manual flash operations, which I prefer. If you prefer to use TTL flash, you're in the wrong place.
Credit where credit is due: I learned of the universal hot shoe translators in this entry of the Strobist blog — which shows the older version and links to a product page that no longer exists (the blog entry is from 2009).
(Note: I do not receive any compensation for mentioning any company or its products in this review.)