There are some cheapo products like this that are meant as marketing materials, and inject something like WinKey-R {some URL} Enter into the keyboard stream to open a Web site. Probably not programmable.

Next up in price are USB numeric pads, mostly just the same as pressing numeric pad keys on a regular keyboard though.

Then there are numeric pads with one or two extra "programmable" keys.

Then there are fully programmable keypads with a grid of 8 to 16 keys, or a "stick" of 10 to 12 in a row to be mounted to the top edge of your keyboard.

Beyond that there are USB devices that accept 4 to 16 (or more) contact closures.


But I have not seen any USB programmable single button device. The closest might be USB "foot pedal" switches. There simply isn't any demand.