OK, I can comment on this one. AFAIK, the 'Green Plug' is a version of the (Frank) Nola power-factor device that came from NASA in the mid '70s. They held the patent, but handed out licenses for production to just about anyone who asked.
The US Department of Energy ate 'em with a spoon big time. Although originally intended only for small appliances, they were later available as an option to then-new electronic [SCR-based] 'soft' motor starters for industrial applications. The snake-oil claims were plentiful.
I tested a pair of them on a package air conditioner in the late '70s by carefully logging daily readings on a dedicated watthour meter connected to the unit. As far as I could tell—at very best—the miracle boxes *added* about 2% to energy consumption. At that time they were often installed for free by plumbing contractors, thrown in with rooftop solar water-heater installations. The particular model connected to my cohort's air conditioner had integral 32-volt (aviation) circuit beakers protecting 230-volt motor circuits—a first-class fire hazard in waiting.
Internally, they looked like they had been built one-at-a-time on someone's kitchen table. They even fooled some building inspectors with a prominent UL (safety) label, but that label applied only to the empty box they were mounted in—not the assembly itself.
More recently, they have resurfaced in mail-order catalogs and demo'ed at hardware stores connected to a small lightly-loaded water pump; hardly an accurate representation of the devices' worth.
Save your money for something truly useful, like flashlight batteries (especially if you live in California.)