The problem, though, is the cheap set-screw boom design of most is cumbersome, hard to use, and error prone.
Instructables user rstraugh came up with Third Hand++, a much improved version using Snap Flow.
I took the same design, and went a step further. The result is the Desk Squid.
Step 1: Construction
I used both 1/4" and 1/2" Loc-Line in the design. The Loc-Line magnetic base, while expensive, eliminated all the complexity of drilling and tapping base plate holes, while adding a lot of versatility. The magnetic bases have rather rough under-surfaces, which when combined with the tremendous magnetic force they exert, can easily scratch most finishes. I found that a bit of fabric and some cyanoacrylate solved the problem nicely.
The magnetic bases are available only in 1/2" and 3/4" sizes, so I got the 1/2" versions, and added 1/2" to 1/4" Y connectors. At the end of each arm, I added a 1/8" right-angle nozzle.
The clip heads were done in exactly the same way as in rstraugh's design. I simply tapped each 1/8" nozzle with a 6-32 tap, and screwed the Radio Shack banana plug into the nozzle. The clips fit over them nicely.
The base is enameled plate steel, to give the magnets something to grip on. I glued a large sheet of black felt (as you would find in any hobby store) to the underside of the steel plate, to protect my desk and provide a nice firm grip.
Before I finished the base, I attached the magnets to the metal frame of my workbench. The magnetic bases are very versatile, and I expect to use them in many unexpected ways.
Step 2: The Future
I rigged up an LED light, but I found it too dim to matter much under the bright fluorescent fixtures in my workbench, so I removed it. I may add one back with a higher performance die eventually. After seeing another Instructable which added LED lamps, I decided to post a picture of it anyway, even though I don't actually use it.
The addition that interests me the most is adding a circuit-board holder of some sort. The jaws of a PanaVise board holder appear to be able to plug into a nozzle on the 3/4" Loc-Line.
At some point, I will probably add a magnifier.
For other ideas, check out rstraugh's original Instructable.