Manfrotto Super Clamps are heavy duty photographic clamps used to attach stuff to stuff. I have two of these, connected by a Manfrotto Adapter Spigot to each other. Depending on what you are going to use them for, you may also need a Stud.
With two of these you can attach light stands to each other, Speedlights to bits of scaffolding, and generally hold things that weren't meant to be held to things which weren't meant to hold them with the kind of security you need when putting expensive cameras and lights at risk.
The quality of these original Manfrottos is superb, and they've never let me down. I got them for using with Brolly adaptors, but ended up using them as well for anything that needed to be done.
The only downside is that they are big and heavy -- more to go in the lighting bag than the camera bag, if you value your back.