Originally made to help Joe McNally solve problems that would occasionally pop up on shoots, the Justin Clamp has become a popular addition to many photographer kit bags. It was created by Bogen/Manfrotto employee Justin Stailey from some standard off the shelf components. It served its function perfectly, so it became known as the Justin Clamp we still have today.
While excellent, they’re not exactly cheap. A handful of these in your bag can set you back a couple of hundred bucks. Another Joe, photographer Joe Edelman, has different solution, though. A DIY option that costs about half of the original Manfrotto (less if you shop around), and it seems to be a very good substitute.