Manfrotto 119 Adapter

We needed to convert a 1/4" thread to 16mm spigot, so we could attach a small light to a Doughty Engineering Knuckle Joint (their code G1155). This adapter was perfect for that - small, relatively cheap, and machined from solid brass. The barrel of the adapter has flat sides, which means that you can get a good 'lock-off' for the thing that you're connecting it too.
I did find other similar products on the web, but they were either more expensive, or not available in single quantities. I'll put a few of these in my tool kit to get me out of a jam in the future.

Good brand, no faults in the product