I've seen variations of it, particularly at flash fiction length, but I guess they're all imitations of this; this story appears to have spawned a whole micro-genre of fiction!
And very funny ending.
Story summary.Kanamit are aliens, recently arrived on earth, & have been offering great gifts to mankind without any expectation of reward: a cheap gadget to generate unlimited power almost for free; a gadget that nearly doubles agricultural yield; a gadget that ensures no explosive of any kind can detonate near it; a treatment that will make humans "taller & stronger & healthier". So we now have plenty of power & food, no war, & soon will be even healthier.
Then a suspicious man steals (& partially translates) what is apparently a handbook called How to Serve Man that all new Kanamit arrivals at their embassy receive as part of their orientation program. And we know they're not quite the altruists we thought they were...
Fact sheet.First published: Galaxy Science Fiction, November 1950.
Winner of Retro Hugo Award 2001 in short story category.
Related: Stories of Damon Knight.